Before getting into the heart of my arguments, it is important to define the terms that I will use going forward. While this material is necessarily dry, I will do my best to make it entertaining.
First up is the tricky matter of defining God. There are almost as many concepts of God as there are believers in God, so I use a definition broad enough to encompass most: God is an omniscient, omnipotent, supernatural intelligence that created the universe and everything it contains (including humans) deliberately. Those who claim a belief in God yet do not subscribe to this definition are, in my opinion, usually closet atheists or agnostics who cannot bring themselves to admit their lack of belief. I will have more to say about them in a later post; for now, I will utilize the definition as stated.
A theist, then, is a person who believes in a God who interacts with the universe, and with humanity in particular. This God (or gods, in the case of polytheism) at least occasionally intervenes in human affairs, and is appropriate to worship (whether or not God is worthy of worship is another matter). In many versions of theism, God provides for some form of an afterlife for at least some humans. Christianity is a theistic religion, as are Judaism and Islam.
A deist is a person who believes in a God who does not interact with the universe. Many central figures in the American revolution, such a Thomas Jefferson, Thomas Paine, John Adams, and James Madison, where probably deists. In those days, deism was the rough equivalent of atheism today, as it represents a rejection of theism. Atheism was less tenable back then (although a strong case could still have been made), because the sciences of cosmology and evolutionary biology did not yet exist to explain the development of the universe and life on earth, respectively. Today, deism is much less popular, and mainly exists as a compromise position between theism and atheism. I will have little more to say about deism, other than to point out that it is functionally equivalent to atheism (there is no reason to worship a deist God, or organize one's life around such a God), and that my many of the arguments in the posts to come will refute deism as well as theism.
The etymologicaly correct definition of an atheist is a person who lacks a belief in God, while the etymologicaly correct definition of an agnostic is a person who believes the existence of God cannot be proven or disproven. I listen to, and recommend, two podcasts from the Atheist Community of Austin (ACA) -- The Atheist Experience and The Non-Prophets -- and they routinely insist, correctly, on these definitions. By these definitions, a fence-sitter who takes no position on the existence of God is an atheist, while the term agnostic applies to nearly everyone.
With apologies to the ACA, I nevertheless prefer the more common definitions of an atheist as a person who believes there is no God, and an agnostic as a fence-sitter. In addition to being common, these definitions are more useful because they define atheism more specifically, and give some utility to the term agnostic. These are the definitions that I will use going forward.
It is important to point out that an atheist is not someone who knows with absolute certainty that God does not exist. Atheism is defined by belief only, and no atheist I know of claims absolute certainty. While I'm sure that, were you to search high and low, you could find such an irresponsible person, it is far more common for theists to claim absolute certainty. Ironically, it is often these same theists who will praise themselves for their humility while accusing atheists of arrogance.
Finally, a fundamentalist is a person who believes in the inerrancy of whatever religious scriptures he or she takes as authoritative -- the Old and New Testaments of the Bible, in the case of a fundamentalist Christian. Religious people who do not believe in scriptural inerrancy are usually referred to as religious moderates or liberals (the difference depending on just how much scripture they are willing to ignore, downplay, or rationalize away). Passionate atheists like me are sometimes pejoratively labeled atheist fundamentalists, but this is just meaningless wordplay, since atheists have no scripture.