The sales invoice is perhaps the most common document in the business world. It is an important record for both the seller and the client, and it's an essential element in the management and bookkeeping of any business.
An invoice is a business document issued any time a sale takes place. Doctors issue invoices after a patient's appointment. Office-supply companies issue invoices when a client places an order.
Invoices serve as a permanent reminder of what was purchased, when it was purchased, how much it cost, who sold it, who bought it and how to reach them. Using the example of the doctor's office visit, an invoice would include the name, address and telephone number of the doctor's office; and the name, address and telephone number of a patient and the services the doctor performed (and charges for each).
Certain businesses deal in time, not in volume. Recording studios, for example, often charge by the hour. So an invoice from a studio would most likely include the number of hours the artist spent recording or working with previously recorded material.
The uses of an invoice are implied in the information they contain. They are a bill calling for payment from the buyer, a proof of purchase for both parties and can be a means of protection for the consumer in that they carry detailed pricing information about the items purchased (making it easier for the buyer to understand the transaction and charges involved).
Traditionally, invoices were sent in the mail or, when the technology became available, via fax. Today, as businesses gravitate toward paperless operations, they are commonly sent via email and filed away on a computer.