An invoice is a document you send to a client that says what they owe, why they owe it, and when payment is due. It is a request for payment, not proof that money already arrived.
What an invoice is for
You use it after you agree the work or the sale. The client uses it to pay you and to keep their own records. You keep a copy so you can see what you billed and when.
What it is not
A receipt confirms payment. A quotation is an offer before work starts. A purchase order is the client asking you to do the work. You can mention a quote or PO number on the invoice so their finance team can match the papers.
Minimum useful contents
The word Invoice, your name, the client name, a unique number, the date, the due date, what you billed, and the amount due. What to include on an invoice lists the full core set.
How InvoiceBizz fits
The free invoice generator builds this document in the browser. Totals update as you type. You download a PDF or print it, then send the file yourself. InvoiceBizz does not email the invoice or collect the payment.