Agnosticism is a philosophical position stating that there can be no proof either way that God exists or doesn't exist. Agnostics believe that there may be a god, but that he/she is ultimately unknowable.
Agnosticism has two general meanings.
One is that knowledge of the divine is impossible. This definition is not to be mistaken with an alternative to atheism. Atheism and theism deal with belief, agnosticism and gnosticism deal with the basis for such belief. For example, agnostic atheism holds that knowledge of the divine is impossible(or currently unobtained) and thus belief in God is unjustified and illogical. On the other hand, agnostic theism is also perfectly valid. Any theist who says ‘just have faith’ is holding such a position, as they are admitting that they have no knowledge of whether a god or gods exist, and yet still believe despite of that. Gnostic atheism and theism are pretty much self-explanatory given that the definition of Gnostic to be ‘believing that knowledge of the existence or nonexistence of the divine is possible or currently held’. A gnostic atheist claims that he knows that god does not exist(or thinks one day such knowledge will be obtained), and thus believes that there is no god. A gnostic theist, on the other hand, believes in god because he claims to know the existence of god.
On the other hand, Agnosticism can be also used in a general sense: not believing in something without evidence. For example, agnosticism about the existence of invisible immaterial pink leprechauns inside computers, making it work. There is no evidence that they do not exist, yet assuming they do exist is almost delusional. As there is no evidence for them, the logical answer is that they don’t exist. The amount of things towards which we are agnostic about is infinite, including the infinite possibilities of extraneous things yes unthought of. Yet believing in such things is not illogical. Thus too, is belief in a god bounded. Unless strict and clear evidence is provided, non-belief is not illogical, in fact, it is the default logical choice.
Also see: Atheist vs. agnostic