The most common and most annoying enemy in the game. They jump up at you and scratch you with their claws. They have a limited attack range and they aren't that tough. Some of your unfortunate scientist friends were unlucky enough to get one of these things on their head. Once a headcrab attaches itself to a human skull, it will turn that person into a zombie.
Zombies are dumb and slow, but they are also tough, and can dish out heavy damage with their claws.
Once a zombie has killed something, it then feasts on the decaying body. You can't bring back your buddies, so put them out of their misery. Houndeye - They may look and sound harmless, and you may want to pet one, but don't. Take them out as soon as possible. They emit a sonic shock wave which can even go around walls.