Retweeting. How it all started and a few Guidelines.

Brief History
Evidence of Retweets are found as far back as 2007, but the "retweet" phenomenon really took off early march 2008, at which point a consensus slowly developed among Twitter users.

For the unitiated user it works this way. "User A which was considered notable by another Twitter user, User B, was copied by User B and reposted under User B's account and prepended with "RT @username", with "RT" meaning retweet. (Wikipedia)

Hashtags in Twitter posts and how to use them.

Hashtags originated in twitter as means to mark keywords or topics in a Tweet. It was created organically by Twitter users as a way to categorize messages, which can be very helpful when searching trough massive amounts of tweets.  Twitters best practice advice is to limit each post to a maximum of 3 tags.

There are also third party tools that use hashtags. eg. http://trendistic.com/ladygaga ,which show graphical trending charts. Another site hashtags.org  shows popular tags.