Can someone answer some questions about Twitter to me?

[Shannon Laforme]

I've just signed up for Twitter (I know how behind the times I am) but am having trouble understanding some things. I immediately started following my fave celeb Sean Patrick Flanery and want to know why there is RT after every one of his comments? Also I see that people ask him questions and he answers them but don't see how or rather where they ask? And why does it say @username right in the middle of a post? Any help would be appreciated as I could not find Twitter for Dummies lol thanks

[Mrs Way]

Rt means retweet and you could ask them a question by saying like for example how are you today @nicolaisawesome and I goes on there profile as a mention I'm quite new aswell