Answer:
Petrol / gasoline in a bottle with a wick. The wick is usually a piece of cloth or some other absorbent material. Often there is a flint (as in a lighter flint) attached to the bottle so the wick can be ignited easily.
The beauty of the Molotov cocktail is its cheapness (even cheaper than the $10 RPG) and the fact that burning liquids can penetrate into even tanks.