Floaters hover through the air by manufacturing and storing noxious gases in their flotation bladders. The gasses that allow them to float are also highly flammable, which makes incredibly vulnerable to fire, but the insulative properties of the gas prevents electrical attacks from harming them. While they are not very intelligent, they move fast and can be difficult to kill, especially if a group of them manages to surround an unwary player character.
Floaters are durable creatures, and move very quickly due to their unique ability to hover. They often travel in groups with super mutants and centaurs, and can pose a very real threat to any player character not prepared for combat.
If normal floaters are bad, then the nasty floaters are far worse. Even more durable and tenacious than their counterparts, they are indeed capable foes. Just like the common floater, they are very susceptible to fire, due to their flammable gasses.
The Art of Fallout 3 book available with the collector's edition of the game includes many versions of floaters but the basic version was not provided in Fallout 3. The floater versions that would have appeared are:
Profiles for floaters, budding floaters (listed as "floater budding") and evolved floaters (listed as "floater evolved") appear in the data files for Fallout: New Vegas, although they do not appear in game, and use the centaur model when viewed in the GECK.