For non-hero units:
can raise, does decay: Normal death of most of the units in the game. They leave a corpse that can be interacted and they are still in the game.
can't raise, does decay: Unit leaves a corpse behind, but it is for visuals only. It can not be interacted (as far as I know). It can not be used by abilities like resurrection, animate dead, cannibalize, meat wagon corpse collect thing, necromancer skeleton thing ... But unit still in the game memory.
can raise, does not decay: This is a bit weird (to me). After completing its death animation, the unit does not leave a corpse and disappears from the game. But in the time window "between" unit takes a fatal damage and starts decaying (Art - Death time determines how long this time window is) unit can still be resurrected with stuff like paladin resurrection or death knight raise dead. Not sure about what else it is used for.
can't raise, does not decay: Unit doesn't leave a corpse behind after its death animation. It disappears from the game. Things like resurrection also does not work during death time.