When an order is placed (SoftLayer_Product_Order::placeOrder), a receipt is returned when the order is created successfully. The information in the receipt helps explain information about the order. It's order ID, and all the data within the order as well.
For PayPal Orders, an URL is also returned to the user so that the user can complete the transaction. Users paying with PayPal must continue on to this URL, login and pay. When doing this, PayPal will redirect the user back to a SoftLayer page which will then "finalize" the authorization process. From here, Sales will verify the order by contacting the user in some way, unless sales has already spoken to the user about approving the order.
For users paying with a credit card, a receipt means the order has gone to sales and is awaiting approval.