You will encounter a variety of different enemies throughout the game. The beginning levels contain less types of enemies in order to introduce you to them gradually. Loading screens give you some information about the enemies.
Walkers - The most common enemy. Easily killable with any weapon or simply by running them over.
Goobers - Exploding enemy that tosses balls of green slime at you. If you run them over the green slime within them causes your vehicle to lose control and slide if you are turning.
Crawlers - Slow moving enemy that is usually found in large groups and will suicidally launch themselves at your vehicle to inflict damage.
Runners - Quick enemy that will latch onto your car and continue to attack until you scrape them off by side-swiping a wall or your side-weapon activates.
Bruisers - High defense enemy which is hard to kill and will deflect your car if you run into them and do not inflict enough damage to kill them. If they get close to your vehicle they will attack it by attempting to eat it.
Chuckers - One armed enemy that throws rocks at your vehicle. When close to you they will swing their arm in order to smash your vehicle.
Beware: Zombies become more powerful as you reach higher missions. While you might easily run over an early mission bruiser if you have a good enough front mounted weapon, on higher levels you'll find that much more difficult.
For information on bosses see the boss page.