What makes a Mod, what makes a good Mod and what exactly should a Mod do?

Im asking this, because with the exception of a few places, some sites are shockingly run. The internet is NOT A DEMOCRACY, so whether you like Mods or not, they are a necessary evil. Unless you personally own your own site, you cannot do anything you want on the net. I personally think this site is very well run with some exceptional Mods, but I think the difference here is that the Mods are firm but fair compared to other places. I don't think they are impartial or biased or play favourites, and the Mods here actually seem to have a personality.

I guess I got off track a bit, but what is the "unofficial" role of a Mod?