Often misunderstood, you will find that an Atheist is merely someone who does not believe in the existence of a deity or deities. The exact opposite of a theist.
Atheists can not be defined by any other terms. They do not follow any belief system or dogma. However, as in most cases in humans, people harboring similar ideas will share certain characteristics. Atheists typically abandon all things supernatural and rely on reason, rationality, evidence, and science as opposed to blind faith.
Militant Atheism is very much an oxymoron. For it is not used to define Atheists who take up arms to fight or kill believers in theism, but is actually used to define people who actively speak up about their ideas. In any other realm of discourse, it would be said that these people have a very strong opinion. But because it is not socially acceptable to criticize religion or religious beliefs, theists automatically turn on the defensive. At this point they find the need to label or demonize the opposition.
Whereas a fundamentalist or member of militant Islam would strike against the opposition with violence and tyranny, a "militant atheist" will fight the opposition with words and ideas.