OOC means any comments that are made "out-of-character." In other words, it is you the player that is speaking and not your character. The general discussion forums are OOC forums.
IC, short for "in-character," refers to anything that your character is saying or doing. All other forums including regions, features, and The Bazaar are considered IC.
If you would like to make an OOC comment in an IC thread, you can do so by encircling your comments in two parenthesis like this:
(( This was a good thread. Thank you! ))
Alternatively, you can also use the OOC button above the posting area. This will automatically put the [ooc] tags around your statement and make it appear in a special out-of-character box. You can also manually type the tag like this:
[ooc]This was a good thread. Thank you![/ooc]