A cannabis concentrate (also called marijuana concentrate, marijuana extract, or cannabis extract) is a highly potent tetrahydrocannabinol (THC) and/or Cannabidiol (CBD) concentrated mass. Marijuana concentrates contain extraordinarily high THC levels that could once range from 40 to 80%, up to four times stronger in THC content than high grade or top shelf marijuana, which normally measures around 20% THC levels. By 2017, distilled concentrate was reportedly available at 99.58% THC content.
Volatile solvents such as ethanol, butane, propane and hexane are often used to prepare extracts, leading to fire and explosion hazards in uncontrolled environments. Super-critical fluid extraction using carbon dioxide alleviates concerns of fire and explosion and results in a high quality product. Different gases are used for extraction due to the difference in properties in the gases we are able to choose from. Determining what gas to use is partially dependent on the polarity, natural state at atmospheric pressure, and shape/composition/molecular weight of the individual molecules. Although these are not the only considerations taken into account when choosing a gas, these are some of the most common. For example, due to butane and propane being hydrocarbons and having a natural tendency to not isolate efficiently, this extraction method is used more for a less targeted extraction. Unless you dial in the parameters on a sub critical to pull lipids, other fats, pigments, undesirable flavonoids, and the chlorophyll in the plant material.
Legally produced concentrates for retail sale in legalized states are often packaged in small lip-balm sized containers. In states like Colorado, because the legality of the cannabis concentrates there are heavy regulations on the containers that hold the concentrate. They must be child- resistant, opaque, and have a multitude of legal text warning the consumer of the risks of consumption. Along with the regulation on the containers that hold the concentrate, the Marijuana Enforcement Division (MED) regulates almost every facet of the cannabis seed to sale process. This of course encompasses the creation or extraction of cannabis extract. 
This article incorporates public domain material from the Department of Justice Drug Enforcement Agency document "What You Should Know About Marijuana Concentrates (Also Known as THC Extractions)".