The same thing as an atheist, but both hate each other because they think that the other is intellectually dishonest.
Do you believe in god?
Agnostic: I don't know... I don't believe the bible is literal. The people who wrote it made some glaring mistakes and got some pretty simple facts wrong, but there's still a chance they got the god thing right.
Atheist: No... there is no god. Religion was obviously a means to control flocks of stupid people, then Christians invented attrition, so now people who believed in religion were smart because they didn't get their necks slashed for not believing in god.