Ah, the Zombie. I loved zombies back before they were so mainstream, and so did Minecraft.
A relatively weak enemy that drops rotten flesh (which can be eaten, in a pinch, but it makes you sick), Zombies are some of the few types of enemies that you'll come across on the surface. Like I said, Minecraft's not really my bag, but I think they only come out at night like in Terraria.
Unlike in Terraria, however, I believe that when daylight comes they burn up.