A method of organizing other users on Twitter. Lists can be either public or private. A public list like the Denver DataMan Chamber List can be seen by anyone.
A comment to a specific person on Twitter. @denverdataman would notify Denver DataMan that a mention had been made about them on Twitter. Sometimes called @ replies because when you reply to someone on twitter you reply with @username.
A method for forwarding a Tweet from someone else to your followers. Done with the command RT. You can read more about the why/how/etiquette of Retweeting in this Mashable article.
An application used to manage Twitter. You can download TweetDeck for free and use it in minutes.