A Goods Despatch Note or GDN is a document used by the supplier's despatch department to send goods. It's raised when they need something shipped, and one copy is kept for their records while sending another over to accounts who will process invoices from there on out to carry out the rest of the operations - without this note being delivered, no customer can get charged (and vice versa). These notes are sequentially numbered, which helps maintain records and find missing notes.
A goods Receipt Note or GRN is used as a receipt by the seller and the customer to validate a successful transaction of the goods. This note is the proof of delivery without which the particular strand of the product supply chain cannot be closed as completed. GRN also acts as the confirmation note in the supply chain that can be matched with the contents on the invoice and the contents delivered.