The Dead in Warm Bodies are unlike the zombies we've come to know and love to fear in our common cultural mythology. These crusty old corpses have a sense of community. A strange one, for sure, as we see when R describes their average day: "We do a lot of standing around and groaning. Years pass this way" (1.1.14). But they also go to church, have their own marriage rituals, and raise children. Children who were bitten and have already become zombies.
In a way, being Dead isn't that much different than being Alive, depending your perspective. Julie tries to tell her dad that zombies are people, too. "We don't understand their thoughts so we assume they don't have any" (2.7.173). A being with a larger consciousness than ours might see our puny little existence in the same way we see R's: shambling around aimlessly, creating religion to try to make sense of it all, finding love in all the wrong places.
The Boneys are the ultimate Dead. They have no skin; they're just walking bones. While it's a little ambiguous as to what makes a Boney, our best guess is that these are the people who "died" while still alive. What we mean is, they gave up on life before their life gave up on them.
These sad skeletons cling to the past, keeping photos to remind them of the inevitability of death. They're basically telling themselves "what's the point of it all if we're going to die anyway?" How very existential. The problem is that their apathy becomes a plague when they try to inflict this method of thought onto others. It spreads, and before you know it, apathy has consumed us all, and no one is living, we're just... existing. And what's the fun of that?