You usually get extra stuff when you pre-order, like beta spots or a nice game box. Nothing special really... It also ensures that you buy the game, before you realize that the game isnt all its hyped up to be and decide not to buy it.
When you pre-order in a store, you pay them 5-10$ and that means when they get their shipment of games in, they'll keep your copy in the store until you come and get it--they won't sell or "run out" unless they get omre pre-orders than their shipment.
When you pre-order online, I don't believe they charge your credit card until the game SHIPS, but when you pre-order online, many times they'll send you something special--like a CD with promotional info, or like Frsblch said, a spot in an online beta.
Personally, I wouldn't bother pre-ordering unless:
1) I KNOW the game will be fantastic.
2) You get something really useful/cool by doing it.
I was thinking that when i preorder a ps2 game onlin at www.link-sang.com that i pay in advance,and when they recieve the game the will send it to me,but i guess i was wrong,good thing i asked first.Thx everyone.