Hey there! :) There are several sources of information about the forums. First, you can always hover your mouse over a section to see a brief description pop up. Second, you also should have received a fairly lengthy private message from one of your Mentors explaining things like this. They are there to help you with any questions.
On the right hand side of the forum is where you will see all of the roleplaying areas, so the terms you are seeing in that column (light, bondage, etc) are categories of roleplays. The small/medium/big groups just refer to how many players are present in the roleplay.
The ons and offs section is where you outline what you like to play and what you do not like to play.
Request threads are housed on the left hand side of the forum, where it says "groups wanted" or "one on ones wanted". :)