What is difference between a invoice and a credit note?

When a trader sells goods or services, he issues an invoice, usually in duplicate, and sends the original to the customer. This is to inform the buyer how much he has to pay. The duplicate is retained by the seller for recording and auditing purposes.

A debit note is sent by the seller to the buyer as an additional invoice when the latter has been undercharged. In contrast, the seller sends the buyer a credit note when goods have been overcharged or when the buyer returns goods.

You can see the debit and credit notes as corrections or amendments to the invoice.