What's with the # and @ on Twitter?

[succubus]

Everything with a @ in front is a Twitter user name so if you want to mention someone you have to write their user name with an @ in front. Users can see all the tweets where their user name is mentioned.

Words with # in front are called "hash tags" they are used to include a tweet in a topic. Users can search for tweets that have a certain hash tag.
One of the most famous hash tags on Twitter was #Egypt when all the protests in Egypt were taking place so people who were tweeting about that subject put #Egypt in their tweets so that the people interested in the subject can read their tweets.
Twitter detects if a certain hash tag (or just some words) appear in many tweets and declares that topic asa "trending topic" sometimes abbreviated TT