Zombie are the weakest mob.(unless you count slimes.) There attacks take away about 2 hearts, but they are slow and die easily.

Ups: they give you rotten flesh which (I don't recommend) can be eaten, but it usually gives you food poisoning. They also give feathers which can be used to make arrows, carrots, and maybe even some iron ingots.

Downs: on hardcore mode, they can break down doors.(only on PC version) Also after you are about lv. 25 or up, they start to wear armor.

Zombie spawn in night or in dark places and die in the sunlight. They make groaning sounds, so at least you will know where they are. They also travel in packs. They will survive during the day if in the rain or in water. Their hits are about the same as a stone sword. Maybe notch wanted that...