In our Judeo-Christian based western society we tend to pigeon-hole categories based on descending dichotomies (well, many times I do anyway. :-) By this, I mean that something is either one or the other: either life or not-life, either pregnant or not pregnant, either a dog lover or a deluded cat lover (don't take this argument too seriously!)
In religious terms, such dichotomy is usually expressed as theist or atheist. While we may have our own individual connotations to these terms, ones that may vary from persuasion to persuation, I should like to use very general meanings. I take these terms to mean this: a theist believes in a god or gods while an atheist does not believe in a god or gods.
I leave it up to you to decide which side of that fence you're on but I'm going to simplify even further and say that a theist believes in God, where God is the set of all possible supernatural/superhuman entities with the ability to take a personal interest in our corporeal plane and the capability to directly influence events in this dimension. So from that, God ranges from the Inuit Raven who hatched mankind in a clam shell, from the pantheon of Roman, Greek and Hindu theologies to the monotheist Jewish Yahweh, the Christian Jehovah and the Islamic Allah.
I would imagine that there are some that would strongly disagree with the above definition, preferring a much narrower definition , perhaps defining theism as only including those fellow travellers who are members of their own religion (eg. Jewish or gentile; people of the Book or not, in Islam). That's fine but I hope you can see what my intent is, in such a liberal definition.
So then an atheist is one who doesn't believe in God.
As is true with many dichotomies, the categories need more refinement. On the theist side, belief is an active process: one positively states, "There is a God". This needs no further categorisation.
However, on the athetist side, a lack of belief in the existence a supernatural may be either active or passive. Just as a theist may actively state, "God exists" an atheist may just as actively state, "There is no God". Both positions are equally philosophically indefensible: for theism, there is no evidence to support their belief but faith; for active or strong atheism, the absence of evidence is not evidence of absence.
Thus, we might consider theism and active atheism as being on the same side of yet another dichotomy, that between logically defensible positions and logically indefensible positions. To me, the logically defensible atheistic position is that of the passive or weak atheist: as there is no evidence of God's existence or non-existence, we cannot know whether or not God does exist. The weak atheist neither belives nor denies the existance of God but merely says, "I do not know". Thomas Henry Huxley, in Victorian England, first labelled this position agnosticism (from the Greek a - without gnosis - knowledge).
Personally, I'm agnostic or a weak atheist. But I'd like to go Huxley one better and sub-divide even further, based on my personal philosophical maunderings.
For a very long while, I just wished the whole issue would simply go away. I neither knew nor cared whether there is a god. This position, I labelled as apatheism. If it weren't for the fact that the beliefs of others continually intrude into my life, I would have been perfectly content there and not have bothered to waste the cerebral ATP in any further consideration.
But the frequent discussions with others have caused me to further refine my position away from the blissful passivity of being an apatheist to take a more active stance in my agnosticism. Not only do I not know whether or not there is a God but now I actively hope that there isn't! I really hope that the supernatural fix isn't in, that the magnificence of both human accomplishments and natural wonders are not the result of a Celestial Design Committee. The universe is, by far, a much more interesting place if the game isn't rigged.
I haven't yet come up with a better term for this position than active agnosticism. While descriptive, it lacks panache. :-) Ah well, one more thing to ponder when the lights are out.