Often abbreviated as RT, retweet is a term used on Twitter that describes the process of mentioning another users tweet. For example, if you read a tweet you enjoyed or think your followers might enjoy you can hover over the tweet and click the Retweet link to create your own tweet. When retweeting a tweet it will begin with RT @computerhope, where @computerhope would be the name of the user you retweeted.
To retweet a user from within Twitter hover over any tweet and then click the retweet link. It is important to realize that the 140-character limitation still applies to retweets, so if you plan on adding more to the retweet you may have to delete some of the characters.