For the examples Alice is your character, and Bob is another character belonging to someone else.
Short for roleplay. Can be used for both the noun (Joining this RP!) or the verb (I'm going to start RPing now)
Short for character. One of your little minions that you have complete dominion over. Mwahahahaha*coughs* Example: "My charrie's still mad that your charrie punched him"
Non Playable Characters. The people that fill the world, give your characters stuff and just generally take up space and breath your character's air. Example: "A supervillain suddenly attacks Alice and Bob from nowhere!"
Taking over someone else's character and deciding what they do, from the extremely obvious (Alice shoved Bob, causing him to stumble away from her and the supervillain) to the less so (Alice picked up Bob and chucked him at the supervillain). Either way, stop it.
Mary Sue Edit
A character who is perfect in every way and succeeds at everything and no. Stop that. Don't do it. Example: "Alice using her powers rewrote the universe so that the villain was never born and everyone got kittens. She flicked back her perfectly shiny blonde hair and her purple colour changing eyes twinkled. "Now that's sorted, would you like to go on a date? ((Don't say no!))" Can also refer to a character who is based off the user playing them to the point that it's just annoying.
Out of Character. This is the talking that you do in the double brackets. Example: ((Hey, what are Bob's powers again)) ((Summoning dogs from another dimension)) ((That's very specific)) ((Better than reality warping)) ((Hey, now you're being mean!)). Ahem.
In character. The place where your character does stuff. Example: Alice sat in the corner of the coffee shop, sulking because Bob wouldn't go on a date with her. She didn't know why he wouldn't, she was beautiful!
Either OverPowered (wow, that sounds like an awesome superhero/supervillain name) or Original Poster. Overpowered is bad. It means you've created a character that won't fit because they've got powers/abilities that obliterate anyone else's role in the RP. Example: "Alice has reality warping powers" "Seriously, that's OP." "No it's not! Alice deserves to be special!".
Meanwhile, Original Poster is value neutral. They are the boss of the thread, the person that made the scenario and handles most if not all of the NPCs in the roleplay and sets everything up. Hopefully the one that makes everyone feel involved. Example: "Hey, OP, can you make a new thread?"
Putting a post on an RP thread so that it shows up in your Recent Forums Post so you can more easily access it later, for if you're on your phone or tablet, or you're just really too busy to make a character now, but you'd really want to later. Example: "This looks really good! Bookmarking!"
Tends to be used for either RPs about canon characters where only one person can have a single character, or RPs with roles that can only be held by a few people, reserving is when you post saying that you're going to put up a form for a specific character later. It's bad form to reserve a character, especially a popular one and not end up actually making it, so most OPs limit reservations for 2 weeks after it was made. Example: "Reserving Deputy Leader of the Random Superhero Squad!"