An invoice is a document issued by a seller to a buyer, detailing the products or services provided. It typically includes information such as the date, a description of the items, quantities, prices, and the total amount due. Invoices serve as a formal request for payment and help keep track of sales and purchases. Invoices are essential for businesses as they provide a record of transactions for both the seller and the buyer. They can also include payment terms, such as due dates and accepted payment methods, ensuring clarity in financial agreements between parties.