FF - Follow Friday. Every Friday users post a tweet with their
favorite users (by "mentioning" them - @username) The idea is that
their other followers will then follow those people in the
tweet.
MM - Music Monday. Every Monday users post a tweet with their
favorite band, artist, song, video etc for other users to check
out.
RT - Retweet. This is when you literally take someones tweet and
post it as a tweet yourself with "RT @username" at the beginning of
it.