"First, a guide to Twitter shorthand. You will see examples of these in the sample tweets that follow:
@username: creates a link to that user in your post.
RT: Retweet, to copy someone else's post in a new update. Give them credit by adding their @username.
#: hashtag, helps to organize your tweets into categories for easier searching.
DM: Direct message, send a tweeter a private message instead of an update that all your followers can read."