An invoice is a commercial document that itemizes products or services provided and requests payment. It includes seller and buyer information, invoice number, date, description of goods/services, quantities, prices, payment terms, and total amount due. Invoices serve as legal records for accounting, tax purposes, and payment tracking. Types include proforma (before delivery), commercial (for customs), and recurring (subscriptions). Digital invoices streamline payments and reduce processing time. Always include clear payment instructions and due dates.