Hello! Welcome aboard to journal roleplaying. Here's a few answers for your questions. I also think that this wonderful site has something akin to a RP FAQ somewhere if you poke around enough!
PSL - Personal Storyline. This is a storyline (game/idea) usually between two people, though it can include more if it's a group storyline. Basically, it's something that's not in a public game.
Celeb community - Here's where I'm not very knowledgeable. As far as I know, yes you play as an actual celeb (without any affiliation toward them). I know that there's a lot of keeping up where your chosen celebrity is in the world, what they're up to and the like.
Examples - I'm assuming 'para' and multi-para' are paragraphs? This depends on what game you choose. Most games I know ask for a solid paragraph or two. Writing length, I think, is a personal matter?
First post - Depends on the game!
AIM - Yes. Though some games also require an OOC AIM for plotting purposes.
Intro - Introduce yourself and your character to a game. Usually it's along the lines of "Hi! I'm name here and am bringing name of character here. Here's a little bit about my character. And here's a little bit about me." The "about me" part is generally timezone, way to get a hold of you..and other tidbits you think you'd like the other players to know.
Thread - It's the roleplaying itself, via the journal. For example, if your questions post was a scene, I'd be answering your "thread" right now. Other ways to play a scene are through AIM, GDocs, or via email.
Faux-Celeb - Creating an original character who is a celebrity. For example, I create a character named Jane Doe and create her personality and a history and so on...then I give her credits. Like, Jane has played in My-So Called Life as Rayanne, Never Been Kissed as Josie, so on. Also not something I personally completely understand so hopefully help will come to you from other people about it.