Standard Zombie

Entity: npc_zombie or npc_zombie_torso

A headcrab's primary goal is to attach to the head of a suitable host using its mouth (typically covering the face, and most of or the whole of the head), and burrow its claws and hind legs into the host, whereupon it takes control over its victim's body, rendering him a mindless zombie-like being.

After a classic (or normal) headcrab attaches to a host, it transforms him/her into a bloody, zombie-like being.

Capabilities
Normal zombies are generally slow, and not very powerful. They limp toward their target and attack it with its claws.

In certain cases, zombies are known to break down wooden barriers and smash objects in their way.

Zombies do not pose much of a threat unless they are in numbers.

Tactics
Normal zombies are quite slow, and can only hurt you by attacking you with their melee attack. Zombies can also do a little bit of damage by throwing objects at you.

One way to defeat a zombie is to equip the crowbar and hit it. It takes about 4 hits to kill the zombie. You can land 2 hits before the zombie can hit you back with his claws.

Another way to defeat a zombie is to shoot the headcrab on its head. It takes about 3 to 4 pistol shots, and ensures the headcrab will not detach and attack you.

It is not very smart using weapons other than the crowbar or pistol to kill zombies because it is normally a waste of ammunition.