There are 4 types of business objects: quote, order, reception and invoice. These 4 types of purchase match the different purchase evolution. The ideal purchase process is as follows: Quote - Order - Reception - Invoice.
However, each type of purchase can be created separately from the other ones. For example, an order can be created on its own, without a quote.
You can also access the billing table from the Purchases module.
To learn more about it, read the following sections: