You get experience points for doing various things. You have a chance of getting experience points every time you log on and every time you vote (either up or down) someone else's post. You also get experience points for making posts that other people vote up, which is where I think most of mine came from.
When I reached level 4 this morning, it told me I had "super-powers" now. It's actually a pretty small super power, but it's a super power nonetheless. Whee.