In technical safety terms, combustibles are harder to ignite than flammables. So diesel and olive oil are combustibles, for example, because neither of them give off enough ignitable vapour at room temperature. Ethanol does, so it gets classified as flammable, and you need to store and handle it more carefully than diesel. Then there’s really horrible stuff like triethylborane which will catch fire upon meeting oxygen even at temperatures well below the freezing point of water

Of course in casual usage they mean the same thing