Theists often assume atheism to be a "belief". And therefore they claim that atheism is a belief system that also requires "faith" (to believe there is no god). This is an erroneous characterization. A "lack of belief" is not a "world view"; it presupposes no rules, doctrine or dogma. It's merely a lack of belief in a god. Do you believe in the non-existence of Santa Claus?Would you call yourself an anti-tooth-fairyian? Is ones' identity or world-view tied to what they don't believe in?
Atheism can be further broken down into the following sub-categories:
Weak Atheist - lacks a belief in god or gods, and acknowledges that it is presumptuous to say for sure there is no god, although in the absence of any evidence, contends it is unlikely and not worthy of consideration
99% of most atheists are the "weak" variety, even though most theists' idea of atheism is something different:
Strong Atheist - believes that there is no God; some argue this position, which is uncommon among atheists, is as equally presumptuous as claiming there is a god (It is accepted that it's virtually impossible to prove a negative, that something does not exist)
A typical atheist may be strong towards a specific deity such as Jesus, claiming that they are confident the Christian god does not exist. Likewise a Christian may be a strong atheist towards the Hindu gods.
Furthermore, agnosticism is a type of atheism. Mainstream media and many people will rarely make note of this because many choose to use the term agnostic to describe their lack of belief due to the negative connotations the media associates with the term atheist, but technically agnostics are atheists: You cannot believe in something you are not sure exists!
Strong Agnostic [atheist] - claims that one can never know whether or not there is a god, or according to Richard Dawkins, they are permanent agnostics in principle (PAP), believing that the existence of a god is unknowable.
Weak Agnostic [atheist] - accepts the idea that there may or may not be a god (in other words, "I don't know, and neither do you.") They believe that the existence or non-existence of god is knowable, but that we do not yet know. Richard Dawkins calls these people Temporary Agnostics in Practice (TAP).
There are some that claim you can be an "agnostic theist", implying someone believes in gods but recognizes that the knowledge of gods' existence may or will never be known. While this is technically true, we are unaware of any practical use of such a term, as the generic definition of [deist] seems to fit that bill. The most common use of agnostic is in an atheistic sense; any theistic use of the term is usually for the purposes of argumentation and rarely represents anyone's particular philosophical view.
True Atheist - A true Atheist (with a capital A) is indicated as an atheist who recognizes that supernatural claims require extraordinary evidence. If you become a true Atheist, you recognize that in the absence of definitive physical evidence of a God or supernatural entity, it is irrational and erroneous to believe in such entities. A true Atheist can not convert back to theism without definitive physical evidence of a supernatural creator.
atheist by default - also known as an apathetic atheist is someone who technically can be described as an atheist because he/she lacks any belief in gods, but has not "arrived" at such a realization through investigation and understanding, instead the atheist by default meets the criteria but ultimately doesn't really care much about the cause or nature of atheism. These people are often fond of calling themselves "atheist" and at the same time calling other atheists out as being annoying and unnecessarily provocative.