As it seems you are mostly having trouble with the section types, I'll go over them quick.
Free
In free, you can expect to find one liners. I do not know much more about the place.
Casual
Casual is the den of posts with usually couple paragraphs of content and some modicum of character development.
Advanced
Anything that is beyond casual, really. Sometimes it goes near the character limit of a post, sometimes less is more, but still rarely under three paragraphs a piece. Character development and compelling storytelling are pretty much a must.
Arena
A fight? Count me in! This place has characters fight each other. Power levels and the attention to detail varies.
Nation
For the emperor! Here people play as whole nations, usually in war against each other AFAIK.
Tabletop
Tabletop roleplaying games through a forum. Simple as that.
1x1
RPs with only two participants. Lots of romance and outright smut here. 18+ content should be played out via PM.
I hope I have been of assistance!