The Messenger: Will quickly dash next to its target, after which is deals standard damage.
The Parted: These zombies have had their bottom halves blown off and drag themselves around. Despite that; they move very fast, but deal very little damage and can't latch on. Being on the ground makes them a harder target to hit.
The Lonely: Nobody likes this poor hobo of a zombie, and he just wants a friend. He deals no damage, but once it grabs on to you it doesn't let go.
The Starved: Very skinny which lowers your chance to hit it, when it attacks you it gains HP. They also won't bother with attacking barricades/doors/windows/etc, if there is a path to a human target they will take it.
The Hidden: These zombies are invisible except for a short period after attacking. Also only attack human targets. This zombie can walk through walls and most other obstacles while hidden.
The Diseased: These zombies move in a cloud of infection, just getting near them will be enough to infect you. If it isn't killed with fire the infection will spread and infect everyone in range.
The Armed: Military personnel recently turned zombie, they still wield machine guns.
The Invincible: Wearing heavy body armor; bullets will just bounce off of these zombies, hitting other nearby zombies or even players. Must be killed with precise melee strikes or fire.
The Exhausted: These zombies have to stop and catch their breath after every few steps, but if they ever do reach you they will exterminate you in a single attack.
The Stand Alone: Standard speed, insanely high HP, and enough attack power to down you in one strike. Once downed they will lose interest in you.