A transaction is considered "closed" once the deeds have been recorded. Then you own the home. However, it is not always possible for you to occupy it immediately. This can happen for several reasons, but the most common is that the seller may be purchasing a home, too. Usually, it is scheduled to close simultaneously with your purchase of their home.
It is sort of like being at a red light when it turns green. Although all the cars see the light change at the same time, the guy at the back of the line doesn't begin moving until all the cars ahead of him have started.
As a result, it has become customary to allow the seller up to a maximum of three days to turn over actual possession and keys to the home. When transfer of possession actually occurs should be clearly laid out in your offer to prevent confusion later.