It all depends on what site you are running. If you are running a DDL site you can earn money via the pay per commission. (When a user clicks a link on your site and buys membership on another site you get paid a fix amount.)

However if you run a forum the main income stream will be donations and PPC. (Adsense/Adbrite/Clicksor). Yes I know adsense isn't allowed to be used when you run a warez forums but I mean a forum in general.