Quote or invoice: what is the difference?

A quote is an offer. It describes the work, sets a price and states how long that price stands. As long as the client has not signed it, nothing is owed.

An invoice is a claim. It records work already delivered and triggers the payment obligation. It is also the document your accounting is built on, which is why its numbering must be sequential and unbroken.

The practical consequence is simple: never send an invoice for work that has not been agreed, and never treat a signed quote as an invoice. The signed quote proves the agreement; the invoice proves the debt.

In practice, a well-written quote becomes an invoice without retyping. Keep the same lines, the same amounts and the same client, change the document type and assign a new number.