An invoice is a request for payment for work that has been done or goods supplied.
What it needs to contain
The business's legal name, address and contact details, and its tax identification where required.
The customer's name and address, matching who is actually liable to pay. An invoice addressed to an individual when the contract was with their company is difficult to enforce against either.
A unique invoice number, sequential, so payments can be matched and nothing is missed.
The date issued and the date payment is due — see net terms.
A description of what was supplied, in enough detail that the customer recognises it. A single line reading "services rendered" invites a query, and a query delays payment.
Amounts, any sales tax shown separately, and any deposit already paid, deducted.
How to pay.
An invoice is not a receipt
An invoice requests payment. A receipt confirms payment was made. They are separate documents and a customer frequently needs both.
A statement is not an invoice either
A statement lists what is outstanding across several invoices. It is a reminder, not a request for a specific piece of work, and it does not replace the invoices it summarises.
Timing
The single largest controllable factor in when money arrives. Work completed on the third and invoiced at month-end has lost weeks before the customer has done anything — see days to get paid.
Invoicing on completion removes that delay entirely.
When it is not paid
See collections and, for work on property, mechanic's lien.
Creating one
See creating an invoice.
Correcting one that has already been issued is a credit note rather than an edit, so the customer's copy and the business's copy continue to agree.