Author(s):

Chris Messina

In 140 characters:

RT is a way of quoting or forwarding someone else's tweet, usually verbatim.

Description:

Retweeting is a simple act for replaying or copying another user's tweet and giving them credit. 

Details and Use Cases:

The retweeting activity has become so widespread on Twitter that the service was altered to include the functionality natively. A special API call and interface is being developed in order to minimized "pollution" of the Twitter stream.