Role playing is basically having a character (yours) interact with other characters (everyone else's), in this case, through text.
OOC means Out of Character. In other words, when you see OOC, generally that person is metioning something that is not happening/has nothing to do with the rp.
IC means In Charcter. That's everything your character does that revovles around the world you're rping.
This is OOC ------> OOC : Hey, it's my Birthday today.
OR
OOC : Is the door to the left, or the right?
OR
OOC : Hey, you can't do that. That can't physically happen.
Here are some examples of IC--------------------> Glancing around, Stacy tiptoes towards the door. Her gun is in her hand, and her knife is tucked safely in her boot.
OR
Jane looks around, and then whispers to Mark, "Is there any way out of here?"
OR
Mark whips out his saber and attempts to stab Jason in the stomach.
EDIT: Darn, Freddy, didn't see you. Oh well...