Many people treat agnosticism as a non-committal approach to the question of Gods existence - this is why it is so often treated as though it were a third way between atheism and theism, with each of the other two committed to some particular answer and agnostics refusing to take sides. This perspective may be mistaken, but it is common enough to merit further explanation.
Just about any position which professes a refusal to commit to either a belief in God or a denial of God will often be considered a form of agnosticism. This non-committal approach is frequently treated as being intellectually and even morally superior to both theism and atheism. The reasons for this are, first, because a refusal to take either position is seen as being open-minded; and second, because both atheism and theism are seen as being dogmatic in a manner that agnosticism avoids.
One of the most common such agnostic positions is to posit that the evidence and arguments on both sides of the debate over the existence of God are basically equal. Neither side as any advantage with anything unambiguously superior; therefore, it would be unreasonable to commit to either side. After all, any logical reasons for committing to one would be counterbalanced by equally logical reasons for committing to the other. The only reasons left would be emotional, thus rendering the position one of emotion rather than reason.
A closely related agnostic position argues that we simply arent in any position to adequately evaluate the evidence offered both for and against the existence of God assuming that any such evidence actually exists. This is closer to the stricter definition of agnosticism in that it focuses primarily upon human limitations as a reason for not being able to arrive at knowledge of the existence of God, but it adds the stipulation that these limitations should also prevent us from believing one way or the other.
One of the problems with this general conception of agnosticism as lack of commitment is that is relies upon a mistaken understanding of belief. Indeed, it arguably relies upon a very Christian understanding of belief because it seems to assume that being either a theist or an atheist requires a person to commit to some proposition through an act of will which carries ethical implications. This makes atheism and theism acts of will for which you can be held accountable thus the supposedly superior morality of this non-committal agnosticism.
Belief is not, however simply a matter of commitment. A person can certainly commit to a cause, an ideal, or an agenda, but beliefs are a bit different. In order to believe something, you dont have to make any sort of commitment. If you believe a proposition, all this says is that your mind accepts that proposition as true. If you do not currently accept that proposition as true, then it necessarily follows that you do not believe it. This doesnt mean that you accept that the proposition is false, either thats a different question.
The point is, however, that while an agnostic might justifiably refuse to commit to any theistic or atheistic agendas, that isnt the same as a refusal to commit to atheism or theism. This means, then, that agnosticism cannot be reasonably regarded as an alternative to atheism or theism in this manner. Agnosticism is a lack of knowledge, not a lack of commitment agnostics still either have a belief in the existence of at least one god or they lack any positive belief in the existence of any gods. The first would make them an agnostic theist while the latter would make them an agnostic atheist.