An invoice is a commercial document traditionally used in business transactions to provide details of sales between a seller and buyer. It lists the goods or services being supplied, their quantities, individual costs, total cost, applicable taxes, and payment terms. For instance, an invoice may include an itemized list such as 'Product A: $50, Service X: $100' with a total due amount and a payment deadline.

Invoices are important for accounting, tax purposes, and record keeping. For example, a company may keep invoices to verify their revenue for the financial year. Typically, they are used within double-entry accounting systems to ensure proper recording of transactions, showing both the credited and debited accounts involved.

Payment terms on an invoice might state "Net 30", which means payment is due within 30 days of issue. Thus, invoices are integral for managing cash flow, financial planning, and reconciling accounts. Businesses often use software, like Easy Month End, to track and manage invoices efficiently, thereby streamlining their month-end close.