Oh man... this is a question for me. To answer your questions in order...
To join a roleplay, usually there is an OOC (Out Of Character) thread that is associated with the RP (RolePlay) with entrance information and requirements. Generally, this is simply a rules page, character submissions, applications, and all the Out Of Character discussion in the RP. These threads are where you ask for permission to enter and do everything that pertains to the RP that has no place in the RP.
The rules of the RP are set by the Author (or Gamemaster [GM] or whatever they wish to call themselves, but the person in charge) and usually fall along these lines:
-Do not Power-play: This means don't control any characters beyond your own. Don't make other people do things or force someone to do something they don't want to do.
-No God-modding: This means don't make yourself a God and be unbeatable. Everyone has faults and everyone has failings. You can't win every fight.
-Be respectful.
-Follow Forum rules.
The Sections and thread headings are descriptors of what are in the RP or really Titles. For example, there is a RP currently going called "Arkane Halls" and that is just it's title. The heading is nothing more than it's name to be honest. If the letters "OOC" are in the title and it is in the OOC board, then it is the Out Of Character board for the RP it describes.
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