IC is short for "In character," and used for roleplay terms. Basically, when someone asks if you're roleplaying IC (or in character,) they'd assume you'll act like the canon character with their personality and such.
"OOC" refers to "Out of Character," and used for roleplay terms. When you're not roleplaying as your character but
rather talking to another roleplayer in either []s or ()s, that is considered "OOC" (or out of character) chat.
EDIT: Blacki, you've beaten me to it! xD