A Molotov cocktail (also called a petrol bomb) is a simple firebomb made by pouring flammable liquid (usually gasoline, alcohol, kerosene and tar) into a glass bottle and stuffing an old rag in the top, then lighting the rag and throwing it at a target. It then explodes in a large ball of fire.
It was first used by Finnish soldiers as an anti-tank weapon during the Russo-Finnish War of 1939-40. The name derives from that of the Soviet Foreign Minister at the time, Vyacheslav Mikhailovich Molotov who had claimed that Russian planes weren't bombing Finland, but rather were dropping food. The Finns offered back some hospitality of their own to greet the Russians.
Because of its simplicity and the availability of the materials necessary, as well as being virtually impossible to detect or neutralize, it is very popular among rioters and insurgents.