It's quite simple, actually. RPing is just like writing on your own, but instead you're coauthoring with the other participants. You can get a better explanation in here than you can ever hope to get from me; I love weaving a tale, but I don't do well trying to explain things. If you think it looks interesting, stop by Kingdom Hearts: The Keyblade War and tell Smoofy that Eternal Dream invited you.
Welcome to KHI, Xblade. Come, join us in the Roleplays to hone your skills as a writer and gather critique from your fellow authors. That book idea of yours sounds interesting; you ever get it published, I'd love to read it. ;p