By putting a @ infront of their username, which is not neccesarily their real name, you are "mentioning" them in a tweet of yours. So, if they click on "mentions" in their profile they will see your tweet.
It will not go directly to their timeline.
Celebs probably get a million tweets a day, so only people you follow appear on your timeline.
If you are thinking of tweeting a celeb...goodluck, pray they click on "mentions" and see your tweet.
Hope this helps.