An invoice is usually the result of a sales process, during which you'd typically first generate a quotation, the customer will accept the quotation, following which you will generate an order, and when the order has been fulfilled, you will generate an invoice for the customer. The customer will pay you on the basis of the invoice. Like a quotation and an order, an invoice can contain multiple components, services and assemblies.

You will typically print or email the invoice, wait for payment, and when it has been received, come back to edit the invoice and set its status to "Payment received".