An invoice is generated, when a service or product is contracually agreed with a client. Both parties are bound to the mutually agreed duties. The prerequisite for the service or product is a purchase order. With the invoice the seller addresses his demand to his client. Another word that can be used for an invoice is a bill.
An invoice has several parts.
Invoices can be paperbased or electronical. Especially in electronic commerce electronic invoices are transfered. EDI standards, as the UN-standard EDIFACT for administration, commerce and transport is to be used.