It is a homemade improvised incendiary device consisting of a glass bottle filled with a flammable liquid, such as gasoline, and a cloth wick which acts as a stopper for the bottle as well as the ignition source.
The molotov cocktail is a single use thrown weapon. The player can carry a total of 6 and it occupies a slot in the left side of the weapons HUD along with other thrown weapons.
In all three Mafia II DLCs they're available for purchase at Giuseppe's shop from the beginning.
You can find more real world information about the Molotov cocktail on its Wikipedia page.
- The cocktail's model is based on a bottle of Humphrey Brandy, which is popular at bars and restaurants throughout Empire Bay.