1. Scientific Atheists - uphold science as the only method of obtaining truths about the world. Believe that the lack of empirical (non-testimonial) evidence is reason enough to not grant assent to the God-claim. Because their disbelief is largely based on the scientific method and statistical reasoning (God isn't impossible, just infinitesimally improbable), their rationale is very difficult to communicate to the religious layman. Richard Dawkins falls into this category.
2. Philosophical Atheists - reason and logic tells them that god is a logical impossibility. Additionally, philosopher-atheists tend to be better at using language than their scientific counterparts. These philosphers can usually make a good case against the religious layman, but when you get them in a room with a philosophical religionist (or apologetic), the conversation quickly spirals into a word game. I consider Bertrand Russell to be a philosophical atheist (although his extensive work in mathematics could place him in the previous category, his ability to use language makes this category more appropriate).
3. Formerly Religious Atheists - strict religious upbringing (Missouri Senate, many Catholics, strong anti-gay churches, etc.), often coupled with antagonistic relations with parents/family causes them to rebel in their adulthood, which takes the form of turning against God. Because their disbelief is just a tool of their angry revolt against their perceived victimizer (God or God through their parents), many of these atheists still feel an internal need for religion, and seem to be the most likely to return to religious belief later in life (as Universal Unitarians). Think Brian Warner (a.k.a. Marilyn Manson), activists, shock-value atheists.
4. Apathetic Atheists - people with no apparent reason for or against disbelief. Atheists of this type seem to have either grown up in atheistic or otherwise non-religious families, or religious belief simply was never able to take root in their minds. Similar to the religious layman, this is the atheistic layman. Frank Zappa is a good example.
Where does your humble author fall in this categorization scheme? I'd say a mix of 1 and 4.