When it comes to the invoice vs. a receipt vs. a bill parallel, you can look at it like this: An invoice is a legal document whose purpose is to authorize a sale; a receipt is a legal document that proves that the sale has been finalized and the product or service paid for. On the other hand, a bill is a general term that does not refer to a specific document in a legal sense. A bill can refer to a number of various documents, but it usually just means a file containing info on how much the customer owes to the vendor.