It's kind of like RPing in the sense that it's a story involving other people. Only, it's not open for people submitting applications and then joining. It's basically a book authored by multiple people. Also, the other people in the project have more say about plot and setting and whatnot.
For example, Shadow, Plaguewalker, and I are currently writing a story together. We knew that we wanted to write a story together, so then we discussed what we wanted and meshed some of our desires together to get the setting. We also worked together to figure out what the general plot was. After that, we created characters to put into this setting and now the story is going where the characters take it.
Some people do collaborations differently, but that's the rough concept--writing a story together with at least one other person.
Off of the top of my head, some of the special rules it would have to have are things like:
1. If you ask to join a collab, the people in it do not have to let you in. Collabs are not RPs and the more people accepted into one, the harder it is to make it work.
2. If you want to collab with someone, ask them in PM. Don't start a thread pleading for someone to collab with you.
3. If the collab is not yours, don't spam it with comments about the story (unless the writers specifically say that's okay). PM those comments.
Hope that helps!