Skeletons or commonly referred to as Boneys are the main antagonists in Warm Bodies and are also the final state all zombies eventually reach after their flesh has completely decayed.
R comments all zombies become them someday.
Bonies are a blackish-gray hue, and lack flesh along with other features humans and zombies have, save for a thin layer of skin. They also lack eyes and ears, but surprisingly seem to navigate better than normal zombies and run at an extremely fast pace without tiring. They were once zombies, but eventually decomposed completely and only their bones were left. They are also shown to be much lighter on their feet than the zombies.
At the beginning of the film R comments says, "They'll eat anything with a heart beat. I mean, so will I, but at least I'm conflicted about it." This suggests they lack any moral centre and are ultimately lost on a human level, being completely zombified. This is evident, as they served as a primary antagonist in the film. Boneys exhibit extremely aggressive behaviour and are feared even by zombies. Hierarchically they seem to possess a superior social status amoung the world of zombies, this is featured largely in the film in behind the scenes footage, and from the Warm Bodies novel.