Since AdWords is a clicks-driven product, we record a user who clicks on your ad and visits your conversion page as a new conversion. If the user later visits your conversion page multiple times within a 30-day period without having clicked on your ad, these visits are seen as multiple ‘transactions.’ 

For example, an additional conversion page visit from a converted user who returns to your site via a browser bookmark would be a transaction.

However, if he or she were to click on your ad again and visit your conversion page, they’d be reported as a new user, and a new conversion.

Thanks Lisa for this!