True Balsamic vinegar is an artisan product from Modena, in Emilia Romagna, Italy, and is made with grape must (juice) that is simmered to make a concentrate, allowed to ferment, then, for a minimum of 12 years, matured in barrels of progressively decreasing size, made from different woods in order to impart different flavours. The result is dark, rich and syrupy and to be used very sparingly.
The real thing will be marked with 'tradizionale' and/or DOC and will be expensive. You can also buy the more afforable, industrially made 'aceto balsamico di Modena', which uses vinegar as well as grape must; as it's not aged for so long, the flavours won't be as strong.
All year round.
Choose the best
For the real deal, always look for the trems tradizionale/DOC or aceto balsamico di Modena. Very cheap balsamic vinegars are just masquerading as either of the above and will have been coloured and flavoured with caramel - although they're fine for salad dressings and glazes, they won't have the authentic intensity of flavour.
In a cool, dark cupboard.
Add just a few drops (connoisseurs use a pipette) of tradizionale to ripe strawberries, slices of well-aged parmesan cheese or very good quality vanilla ice cream. Brush aceto balsamico di Modena over roasting chicken or duck breasts, shake some over grilled tuna steaks, drizzle over tomato salads or stir a little into a roast vegetable pasta sauce.