Currently there are five different zombies.
'Normal' - They look different, depending on what map you're on. The most common.
Fast - They have a few different looks, and are found on most maps, but are less common.
Poison - They have two different looks, and are found on most maps. Even less common
Shade - They are found in dark maps, or dark locations. They make no sound except for walking, death and attacking.
Guard - They are found mostly in buildings. They are armored and have more health.