An invoice is a formal document issued by a seller to a buyer that outlines the details of a transaction for goods or services provided. It typically includes the invoice number, date, seller and buyer information, a description of the products or services, quantities, unit prices, total amount due, payment terms, and due date. Invoices serve as a request for payment and are crucial for maintaining accurate financial records. They help businesses track sales, manage cash flow, and support accounting and tax reporting. Invoices can be sent electronically or on paper and often include tax details such as sales tax.