You are charged when it becomes available for downloading, not at the time of pre-ordering (between pre-ordering and it being released you could cancel the order, so you are only charged when it's released).
When the pre-order item becomes available, your account will be billed for the pre-order, and the item will appear in the Purchased section of your account.
Some pre-orders include songs that are available for download immediately. When you pre-order, you will be charged only for these songs and you will receive them immediately.
If it's an album with songs that you can download before the album's release then you are charged for the pre-release songs as they become available, and on the album's release you will only be charged for the album's remaining value i.e. the album total price minus any songs that have already been released.