A cart is a temporary holder for a collection of items to be ordered plus any order-fulfillment data (contact/shipping info, payment). The contents of a cart may be used to create a new order, or may be abandoned. They cannot currently be added to an existing order, although this might be useful functionality.
Cart functionality is defined in:
- shop -- customer-facing shopping support
- cart -- basic business logic
- cart/cart-lines.php -- handles content items