Agnostics fall into two categories, those who believe that it is impossible to determine whether or not there is a God, and those who don't care enough to form an opinion.
The first kind think this means they'll still make it to heaven (if god exists-hooooo!) when they die because they didn't doubt God (<--wut), yet still "win" religious debates on forums by 13 year old boys by saying the other side is closed-minded.
The second type of agnostics argue that because one cannot prove or disprove the existence of God, afterlife or metaphysics, that there is no reason for them to adopt a stance either way.