In English, a phrasal verb is a phrase such as turn down or ran into which combines two or three words from different grammatical categories: a verb and a particle and/or a preposition together form a single semantic unit. This semantic unit cannot be understood based upon the meanings of the individual parts, but must be taken as a whole. In other words, the meaning is non-compositional and thus unpredictable. Phrasal verbs that include a preposition are known as prepositional verbs and phrasal verbs that include a particle are also known as particle verbs. Additional alternative terms for phrasal verb are compound verb, verb-adverb combination, verb-particle construction, two-part word/verb or three-part word/verb (depending on the number of particles) and multi-word verb.
There are at least three main types of phrasal verb constructions depending on whether the verb combines with a preposition, a particle, or both. The phrasal verb constructions in the following examples are in bold.
When the element is a preposition, it is the head of a full prepositional phrase and the phrasal verb is thus prepositional. These phrasal verbs can also be thought of as transitive and non-separable; the complement follows the phrasal verb.
a. Who is looking after the kids? – after is a preposition that introduces the prepositional phrase after the kids.
b. They picked on nobody. – on is a preposition that introduces the prepositional phrase on nobody.
c. I ran into an old friend. – into is a preposition that introduces the prepositional phrase into an old friend.
d. She takes after her mother. – after is a preposition that introduces the prepositional phrase after her mother.
e. Sam passes for a linguist. – for is a preposition that introduces the prepositional phrase for a linguist.
f. You should stand by your friend. – by is a preposition that introduces the prepositional phrase by your friend
Verb + particle (particle verbs)
When the element is a particle, it can not (or no longer) be construed as a preposition, but rather is a particle because it does not take a complement. These verbs can be transitive or intransitive. If they are transitive, they are separable.
a. They brought that up twice. – up is a particle, not a preposition.
b. You should think it over. – over is a particle, not a preposition.
c. Why does he always dress down? – down is a particle, not a preposition.
d. You should not give in so quickly. – in is a particle, not a preposition.
e. Where do they want to hang out? – out is a particle, not a preposition.
f. She handed it in. – in is a particle, not a preposition.
Many phrasal verbs combine a particle and a preposition. Just as for prepositional verbs, particle-prepositional verbs are not separable.
a. Who can put up with that? – up is a particle and with is a preposition.
b. She is looking forward to a rest. – forward is a particle and to is a preposition.
c. The other tanks were bearing down on my Panther. – down is a particle and on is a preposition.
d. They were really teeing off on me. – off is a particle and on is a preposition.
e. We loaded up on Mountain Dew and Doritos. – up is a particle and on is a preposition
f. Susan has been sitting in for me. – in is a particle and for is a preposition.
The aspect of these types of verbs that unifies them under the single banner phrasal verb is the fact that their meaning cannot be understood based upon the meaning of their parts taken in isolation: the meaning of pick up is distinct from pick; the meaning of hang out is not obviously related to hang.
Distinguishing phrasal verb types
When a particle verb is transitive, it can look just like a prepositional verb. This similarity is source of confusion, since it obscures the difference between prepositional and particle verbs. A simple diagnostic distinguishes between the two, however. When the object of a particle verb is a definite pronoun, it can and usually does precede the particle. In contrast, the object of a preposition can never precede the preposition.
a. You can bank on Susan. – on is a preposition.
b. *You can bank Susan on. – The object of the preposition cannot precede the preposition.
a. You can take on Susan. – on is a particle.
b. You can take Susan on. – The object of the particle verb can precede the particle.
a. He is getting over the situation. – over is a preposition.
b. *He is getting the situation over. – The object of the preposition cannot precede the preposition in the phrasal verb.
a. He is thinking over the situation. – over is a particle.
b. He is thinking the situation over. – The object of the particle verb can precede the particle
Thus the distinction between particles and prepositions is made by function, because the same word can function sometimes as a particles and sometimes as a preposition.
Some notes on terminology
The terminology of phrasal verbs is inconsistent. Modern theories of syntax tend to use the term phrasal verb to denote particle verbs only; they do not view prepositional verbs as phrasal verbs. In contrast, literature in English as a second or foreign language ESL/EFL, tends to employ the term phrasal verb to encompass both prepositional and particle verbs. 
Note that prepositions and adverbs can have a literal meaning that is spatial or orientational. Many English verbs interact with a preposition or an adverb to yield a meaning that can be readily understood from the constiuent elements.
Furthermore, the same words that occur as a genuine phrasal verb can also appear in other contexts, as in
1(a) She looked up his address. Phrasal verb.
1(b) She looked his address up. Phrasal verb.
2(a) When he heard the crash, he looked up. Not a phrasal verb.
2(b) When he heard the crash, he looked up at the sky. Not a phrasal verb.
The terminology used to denote the particle is also inconsistent. Sometimes it is called an adverb and at other times an intransitive prepositional phrase. The inconsistent use of terminology in these areas is a source of confusion over what qualifies as a phrasal verb and the status of the particle or a preposition.
Concerning the history of the term phrasal verb, Tom McArthur writes:
The value of this choice and its alternatives (including separable verb for Germanic languages) is debatable. In origin the concept is based on translation linguistics; as many single-word English and Latinate words are translatable by a phrasal verb complex in English, therefore the logic is that the phrasal verb complex must be a complete semantic unit in itself. One should consider in this regard that the actual term phrasal verb suggests that such constructions should form phrases. In most cases however, they clearly do not form phrases. Hence the very term phrasal verb is misleading and a source of confusion, which has motivated some to reject the term outright.
The aspect of phrasal verb constructions that makes them difficult to learn for non-native speakers of English is that their meaning is non-compositional. That is, one cannot know what a given phrasal verb construction means based upon what the verb alone and/or the preposition and/or particle alone mean, as emphasized above. This trait of phrasal verbs is also what makes them interesting for linguists, since they appear to defy the principle of compositionality. An analysis of phrasal verbs in terms of catenae (=chains), however, is not challenged by the apparent lack of meaning compositionality. The verb and particle/preposition form a catena, and as such, they qualify as a concrete unit of syntax. The following dependency grammar trees illustrate the point:
The words of each phrasal verb construction are highlighted in orange. These words form a catena because they are linked together in the vertical dimension. They constitute units of meaning, and these units are stored as multi-part wholes in the lexicon.
A complex aspect of phrasal verbs concerns the distinction between prepositional verbs and particle verbs that are transitive (as discussed and illustrated above). Particle verbs that are transitive allow some variability in word order, depending on the relative weight of the constituents involved. Shifting often occurs when the object is very light, e.g.
a. Fred chatted up the girl with red hair. – Canonical word order
b. Fred chatted her up. – Shifting occurs because the definite pronoun her is very light.
c. Fred chatted the girl up. - The girl is also very light.
d. ?Fred chatted the redhead up. - A three-syllable object can appear in either position for many speakers.
e. ??Fred chatted the girl with red hair up. – Shifting is unlikely unless it is sufficiently motivated by the weight of the constituents involved.
a. They dropped off the kids from that war zone. – Canonical word order
b. They dropped them off. – Shifting occurs because the definite pronoun them is very light.
c. ??They dropped the kids from that war zone off. – Shifting is unlikely unless it is sufficiently motivated by the weight of the constituents involved.
a. Mary made up a really entertaining story. – Canonical word order
b. Mary made it up. – Shifting occurs because the definite pronoun it is very light.
c. ??Mary made a really entertaining story up. – Shifting is unlikely unless it is sufficiently motivated by the weight of the constituents involved.
Shifting occurs between two (or more) sister constituents that appear on the same side of their head. The lighter constituent shifts leftward and the heavier constituent shifts rightward, and this happens to accommodate the relative weight of the two. Dependency grammar trees are again used to illustrate the point:
The trees illustrate when shifting can occur. English sentence structures that grow down and to the right are easier to process. There is a consistent tendency to place heavier constituents to the right, as is evident in the a-trees. Shifting is possible when the resulting structure does not contradict this tendency, as is evident in the b-trees. Note again that the particle verb constructions (in orange) qualify as catenae in both the a- and b-trees. Shifting does not alter this fact.
Similar structures in other languages
Phrasal verbs are represented in many languages by compound verbs. As a class, particle phrasal verbs belong to the same category as the separable verbs of other Germanic languages. For example in Dutch, de lamp aansteken (to light the lamp) becomes, in a principal clause, ik steek de lamp aan (I light the lamp on). Similarly, in German, das Licht einschalten (to switch on the light) becomes ich schalte das Licht ein (I switch the light on).
A few phrasal verbs exist in some Romance languages such as Lombard due to the influence of ancient Lombardic: example fa foeura (to do in: to eat up; to squander) and dà denter (to trade in; to bump into) in Lombard. Some of these verbs are used also in standard Italian, for instance "far fuori" (to get rid of), "mangiare fuori" (to eat out) and "andare d'accordo con" (to get on/along with).
An extension of the concept of phrasal verb is that of phrasal noun, where a verb+particle complex is nominalized. The particles may come before or after the verb.
standby: We are keeping the old equipment on standby, in case of emergency.
back-up: Neil can provide technical backup if you need it.
onset: The match was halted by the onset of rain.
input: Try to come to the meeting – we'd value your input.
If the particle is in first place, then the phrasal noun is never written with a hyphen, if the particle comes second, then there is sometimes a hyphen between the two parts of the phrasal noun.
The two categories have different values. Particle-verb compounds in English are of ancient development, and are common to all Germanic languages, as well as to Indo-European languages in general. Those such as onset tend to retain older uses of the particles; in Old English on/an had a wider domain, which included areas now covered by at and in in English. Some such compound nouns have a corresponding phrasal verb but some do not, partly because of historical developments. The modern English verb+particle complex set on exists, but it means "start to attack" (set itself means start a process). Modern English has no exact verbal phrase equivalent to the older set on, but rather various combinations that apply different nuances to the idea of starting a process—such as winter has set in, set off on a journey, set up the stand, set out on a day trip, etc. Verb-particle compounds are a more modern development in English, and focus more on the action expressed by the compound. That is to say, they are more overtly verbal.
^That unpredictability of meaning is the defining trait of phrasal verb constructions is widely assumed. See for instance Huddleston and Pullum (2002:273) and Allerton (2006:166).
^Concerning these terms, see McArthur (1992:72ff.).
^Declerck, R. Comprehensive Descriptive Grammar of English, A – 1991 Page 45 "The term multi-word verb can be used as a cover term for phrasal verbs, prepositional verbs, prepositional phrasal verbs and combinations like put an end to."
^The Collins Cobuild English Grammar (1995:162) is a source that takes prepositional verbs to be phrasal verbs. Many other grammars, in contrast, distinguish between prepositional verbs (the additional word is a preposition) and phrasal verbs (the additional word is a particle).
^Ron Cowan – The Teacher's Grammar of English: A Course Book and Reference 2008 Page 176 "The Adverb Insertion Test – Earlier, we saw that intransitive phrasal verbs usually do not permit the insertion of an adverb between the verb and the particle, and the same is true of transitive phrasal verbs, as (25a) and (25b) show. In contrast, prepositional verbs do permit adverb insertion, as (25c) demonstrates. (25) a. He turned quickly out the light. = separable phrasal verb. b. He ran unexpectedly into his cousin = inseparable phrasal verb. c. He stared intently at the target = prepositional verb. The Relative Clause Test Relative clauses in which the relative pronoun is the object of a preposition permit the two patterns shown in (26). (26) a. The man [that they were waiting for] was late b. The man [ for whom they were waiting] was late. In (26a), the preposition for is at the end of the relative clause enclosed by square brackets, but (26b) shows that this preposition can also occur at the beginning of the clause before the relative pronoun whom."
^For a list of the particles that occur with particle phrasal verbs, see Jurafsky and Martin (2000:319).
^Jeanette S. DeCarrico The structure of English: studies in form and function – Volume 1 – Page 80 – 2000 "4.6.3 Prepositional Phrasal Verbs – It is also possible to find phrasal verbs that are themselves followed by a preposition. These structures are called prepositional phrasal verbs or multiword verbs. Examples are put up with (e.g., I can't put up with) "
^For an example of the shifting diagnostic used to distinguish particle verbs from prepositional verbs, see Tallerman (1998:129).
^Concerning the difference between particles and prepositions with phrasal verbs, see Jurafsky and Martin (2000:318).
^For examples of accounts that use the term phrasal verb to denote particle verbs only (not prepositional verbs), see for instance Tallerman (1998:130), Adger (2003:99f.) and Haiden (2006).
^For example, the series 'English File' uses phrasal verbs in this way. This exercise on the English File website features both types of verbs under the term "phrasal verbs". elt.oup.com