Don't smoke when you're getting gas at the gas station! Gasoline is flammable, as is dry grass, old wood, cloth and mattresses.
Flammable is a pretty straightforward word with one odd fact about it. The word, inflammable, which sounds like the opposite of flammable, actually means the same thing — easily catching on fire. If you want to say that something won't catch fire easily, use fireproof and you'll be sure to be right.