Zombies burrow into the ground and surprise attack you from beneath on anything that isn't a floating platform. They tend to spawn in groups and deal 10-20 damage while having some high health.
It will walk from side to side of the floor until it spots a player. It cannot jump, but will fall through blocks in order to reach a player below them.
Zombies can go inside the ground rapidly and move to any area they can attack you pretty quickly for 10-20 damage.
The best way to kill a Zombie other than spike traps is to hit it and then jump above it over and over until it dies.