Anti-religious philosophy that can be divided into two primary factions:
1. Healthy Atheists. These people simply don't follow any particular religious faith and generally don't concern themselves with attempting to prove to the religious that nonbelief is the way to go.
2. Militant Atheists. These are the guys that take Atheism and turn it into a zealous religion that is focused almost entirely on defaming, badmouthing, and attempting to disprove most or all religious faiths, although in many cases the individual Militant Atheist will focus only on one particular religion to direct their stream of hate at. They appear to be roughly as populous as fundamentalist Christians are on the Internet, and it appears that these two groups imagine themselves to be in mortal combat with one another, judging by some of the crap found on their sites.
Many Atheists don't appear to disbelieve in God so much as they appear to wish agonizing death upon Him and His followers.