A purchase invoice is the supplier's bill to you. Where a (sales) invoice is something you send to a customer, the purchase invoice is something you receive and have to pay. When it is posted, the cost/purchase, the VAT and your debt to the supplier are recorded.
How it fits into the purchase
The purchase invoice usually matches a goods receipt: you have ordered (purchase order), received the goods (receipt) and then get the bill (purchase invoice). You can create the purchase invoice directly from the receipt, so figures and goods carry over.
Credit note from the supplier
If the supplier returns money or goods, you receive a credit note that reverses the purchase invoice — just like a credit note on the sales side.
Attachments
You can attach the PDF document itself to the purchase invoice, so the documentation stays with the posting.
Read more in Purchase invoices and Purchase invoice (quickguides).