on twitter i often see this @ and then a name.What is that?

[Nikos A]

and how can i use it?

[Chris Love]

it is a reply to another user or just indicating another user. So if your Twitter name is JohnSmith you can 'reply' to them or indicate them in your twit by prefixing the @ to JohnSmith, @JohnSmith. This is an indicator to Twitter to link that part of the message to JohnSmith's profile. Twitter clients like Witty, etc will also play off that to allow you to reply or direct message the user.
The syntax to send a Direct message would be D JohnSmith.
If you want to retweet a tweet, then simply start it with rt.