@: At reply. A public tweet directed at a fellow Twitterer, such as @Barack Obama, that shows up in their Twitter stream.

DM: Direct Message. A private message that appears in a Twitter inbox. You can only direct message people who follow you.

RT: Retweet. A tweet that you like so much that you are resending to your followers. Usually includes credit to original tweeter, such as RT @BarackObama, followed by the tweet.

Whale Icon: The iconic blue whale that pops up when Twitter is down. It appeared frequently in Twitter’s first year and a half.

#: Hashtag. Used to designate a topic such as #SanDiegoFire so that people can easily search for tweets on a topic. (It is totally unnecessary, though, because a search on a keyword without the # returns the same results).

Nudge: A feature that lets you send a note to a Twitterer encouraging them to tweet more frequently. You can only nudge people who are tweeting from a mobile phone.