A zombie is a corpse that has risen from the dead. Most zombies are created through viral experimentation, radiation, or supernatural forces. In some cases, someone who has died will re-animate no matter how they have died.
Zombies have been made famous through film, television, comic books, novels, video games, and so much more. As zombies are already dead, you can't really "kill" them so to speak. Shooting them in the head will destroy them. Keep in mind that they don't feel pain. Don't try kicking them in the balls. It doesn't work. Stay safe. Stay alive.