An order in Ãœbercart represents a single transaction. Orders are created during the checkout process where they sit in the database with a status of In Checkout. When a customer completes checkout, the order's status gets updated to show that the sale has gone through. Once an order is created, and even during its creation, it may be acted on by any module to connect extra information to an order. Every time an action occurs to an order, hook_order() gets invoked to let your modules know what's happening and make stuff happen.