Twitter help with commands?

[J]

I have just started using twitter and I have noticed when people mention others names they sometimes put a @ infront of there name and sometimes a # for example.
@John
#John

Also what does these mean?
#FF
#RT


Thanks.

[Andy G]

@John = a reply to another user on twitter.

RT= Retweet, used when a user reposts another user's tweet.

#FF = A hashtag ( #) is a tag that allows users to search topics in tweets (sometimes people will put a hashtag in front of a band or tv show they're tweeting about.), and FF stands for Follow Friday, used to recommend other users on fridays!

[Sara]

Hi................ I completely agree with Andy g, good answer described for Twitter's symbols.