the first one takes place and participates in the debate between theists and atheists, the agnostic stance consisting in the suspension of judgment.
Typically (and certainly historically) the theists put forward the claim "There exists a God, and He has the following characteristics...".
Then the atheists deny that such a God exists.
The agnostics accept the definition of "God" but do not side with either the theists or the atheists.
And then there is another sort of agnostic, but "agnosticism" in this case is but an euphemism for intellectual laziness. Such an agnostic refuses to participate in the debate and his or her stance can be summed up as follows:
As for "God" : anything goes
I do not know anything
I do not care
The essential difference between the two is that the former takes the debate seriously by accepting the common ground of this debate: the notion of God that is being asserted by the theist and denied by the atheist. In the absence of such a common ground, a meaningful discussion is impossible. A person who calls him- or herself "agnostic" should accept this common ground, or they should stay out of the debate altogether.