When you are creating an invoice you will see two options at the top and bottom of the page: Preview and Save.
Clicking Preview will show you what your customer will see, even though you haven't actually created the invoice yet. Clicking Save will then create a draft.
At this point the information on the invoice has been saved, however, it's still only a draft, as indicated by the status in the upper right. Draft invoices cannot be sent to your customers, nor do they reflect in your income reporting.
Clicking Edit Draft will allow you to make any additional changes needed prior to turning it into an actual invoice. When you're ready to turn your draft into an invoice, click Approve Draft.
You've now created your invoice! Note that the status has changed to 'UNSENT'. You now have options to:
'Edit Invoice' to make any changes needed prior to sending to your customer
'Send Invoice...' to send your invoice to your customer
'Mark as Sent' to change the status to 'SENT' without actually emailing the invoice
You can also click on More Options to:
Duplicate the invoice (make a new invoice using this one as a copy)
Make a recurring invoice using this one as a template
Export a copy of this invoice as a PDF
Get a unique URL where this invoice can be viewed online