This category lists materials (including liquids like Tar), blocks and objects that can be set on fire.
It does not include objects that can set flammable materials on fire themselves or operate with open fire (like torches, campfires, fire pits, fireplaces etc.) (usually to be found in the Category that lists heat-emitting objects), nor Fuel for Forges, nor any materials/blocks that can be transformed by being heated, like by throwing Fire Bombs or Flaming Skulls.
In Creativerse, all flammable stuff vanishes very quickly after starting to burn. As soon as flames start to appear on/instead of an object, you cannot pick up this burning object any longer in order to save it.
When placing flammable objects like wooden ones close to (or especially directly above) torches or any other items with open flames or fiery blocks like Hardened Lava, flammable objects can easily catch fire and burn to nothingness in hot environments that make a heat meter show up, like Oceans, Jungles or the Lava layer, but also in much cooler Swamplands.
Placing several torches or heat sources with open flames next to each other can even raise the chances to set flammable objects on fire. Liquid Lava, Fire Bombs or Flaming Skulls will immediately set flammable objects on fire.
If flammable objects start burning, the flames are then able to spread to other nearby flammable blocks, even across gaps of 1-2 blocks on game worlds and player claims where the "fire spreading" option is enabled. This way a whole wooden building or forest (especially Wildwood, Parchwood and Shorewood) can burn down in the worst case.
Fire cannot be extinguished by using liquids, but it can be stopped from spreading by claiming the area and making sure that the claim (advanced) option "fire sim/spreading enabled" is disabled (by default). Fire Bombs (and Flaming Skulls) will have no effect on these claims either.
It is also possible for owners of the game world (F2P players included) to toggle the spreading of fire on a whole game world in the basic world options ("edit world" and "disable fire spread"), which will also render Fire Bombs and Flaming Skulls useless there. Another option to stop fire from spreading would be to create forest aisles/swaths wide enough so that the flames cannot leap over.