I'd been wondering for a while what MT, PRT, and HT mean on Twitter.

Now I know:

MT = Modified tweet. This means the tweet you're looking at is a paraphrase of a tweet originally written by someone else.

RT = Retweet. The tweet you're looking at was forwarded to you by another user.

DM = Direct message. A direct-message is a message only you and the person who sent it can read. IMPORTANT: To DM someone all you need to type is D username message.

PRT = Partial retweet. The tweet you're looking at is the truncated version of someone else's tweet.

HT = Hat tip. This is a way of attributing a link to another Twitter user.

CC = Carbon-copy. Works the same way as email.