The core difference
An invoice is sent before payment. It tells the buyer what they owe and when it is due. A receipt is issued after payment. It tells the buyer that the transaction is settled.
Think of an invoice as the question — "Can you pay me?" — and a receipt as the answer — "Yes, paid in full."
Side-by-side comparison
Do you need both?
In many freelance and small business contexts, yes. The invoice creates the paper trail that money is owed; the receipt closes it once payment arrives. Both can be important for tax filing, expense claims, and dispute resolution.
What about a proforma invoice?
A proforma invoice is a preliminary document sent before goods or services are delivered. It looks like a regular invoice but is not a payment demand — it is closer to a confirmed quote. It becomes a real invoice once the work or delivery is complete.