1. While you're creating an invoice you will see two options at the bottom of the page: Save as draft and Save.
2. If you click Save as draft, the invoice will be saved as you have created it, but no figures will be entered into your accounting. On your Invoice page you will see it listed as a draft. You cannot send this until you save it as an invoice, but you CAN download it as a PDF. You could use the Save as draft option if, for example, you began creating an invoice for a customer but that customer has yet not completed his/her order. An actual invoice is not created when you choose this option.
3. When you're ready to turn this draft into an invoice, click Edit...
...and the Save.
From here you can Add a payment, or Send the invoice to your customer.
4. On your Invoices page, you will see this invoice status will show as Saved until you Send it or mark it paid.
And that is the difference between Save a draft and Save.