When a zombie or animal is killed its body will decompose into a mesh of bloody flesh and gore. Zombie gore has 3 stages, in the first one the zombie corpse is decomposed to half of the zombie's body with its ribs showing and leaking blood as well as some bones lying around. In its second stage, all the flesh decomposes and it turns into half of a blood-covered skeleton. In its third and last stage the body turns into a pile of red decomposed gore and guts. If other zombies die and decompose on top of these gore blocks they will pile up allowing other zombies to climb and walk over them and overcome the player's defenses this way if not removed.
Animal gore has only 1 stage. Its body decomposes into a mesh of guts and organs within a rib cage, in this case if the gore block is not removed it will attract Spider Zombies.
Gore will biodegrade and therefore vanish automatically after a longer time.