1. Obsolete Definition = Hatred of, or rebellion against a particular god.
2. Street Definition = The belief that a god or gods do not exist.
3. Philosophical Definition = The lack of belief in a god or gods; equivalent to non-theism.
The philosophical definition is usually broken down into two sub-groups:
3a. Strong Atheism = Equivalent to the street definition, #2 above.
3b. Weak Atheism = Emphasized the lack of a positive belief. Equivalent to "agnostic atheism".
1. Bad Definition = Undecided about the existence of a particular god or gods. Someone who is 50-50.
2. Street Definition = Equivalent to weak atheism.
3. Philosophical Definition = Says that we don't know anything about a god or gods.
Likewise this last definition can be broken into two sub-groups.
3a. Strong Agnosticism = The position that it is impossible to know about a god or gods.
3b. Weak Agnosticism = The position that one doesn't know about a god or gods, but such knowledge might be possible.
I think most arguments are between people supporting either the street definitions or philosophical definitions. These arguments are similar to the old descriptivist/prescriptivist debate, and also remind me of the hacker/cracker arguments as well.
The second layer of confusion added on top of these disputes is the definition of "god". Depending on what definition of god we are talking about, my own label can change drastically. For example, if your god is literally the universe, then I am a theist, since obviously the universe exists. If your god is some vague deistic first cause, I am either an agnostic or ignostic. If your god is Yahweh, Allah, or Quetzalcoatl then I am an atheist.
In any discussion it is important to get your meanings straight so everyone knows exactly what everyone is talking about. This advice is even more important when it comes to talking about religious beliefs, or lack thereof.