There were two Latin words that were very similar: flammare which means "to set on fire" and inflammare which means "to be caused to be set on fire".
My understanding was that flammare was the deliberate act of setting something on fire, whereas inflammare was more for just the state of being set on fire. So a house could be caused to be set on fire by a chip pan accident, lightning strike etc, but a torch or candle would more likely just to be set on fire.
Something like that, anyway.
These days, however, the two words do tend to mean the exactly the same.
I've heard one distinction between them being that "flammable" simply means "can be (deliberately) set alight", whereas "inflammable" means "can catch fire (with no deliberate act involved)". Maybe this is correct, but I'm not sure.