How does pre-ordering a game on the internet work?

I need to know because I want to pre-order Pokemon diamond for DS, If I pre-order am I guaranteed to get one shipped to me? Or will I have to pick it up at the game store?How does pre-ordering a game on the internet work?depends where you preorder it from, at certain stores they reserve it for you and you have to go pick it up. other stores they ship it to you