The weakest enemy you will encounter in Killing Floor. They often appear in groups with one or two Bloats on their tail. Clots are the most common enemies, occurring in all the waves and usually in the largest number. While they may not seem dangerous, their numbers may overwhelm players who try to combat them. However, the main danger of Clots lies in their habit of distracting players from the more powerful and dangerous specimens.
Extract from Wikipedia.