A domain extension is the part of a domain name that you purchase. For example this can be .org or .co.uk or .com.au.
As of September 2018 there are 1540 total Top Level Domains available, though a few have been retired and can no longer be registered.
Take a look at our domain registration page to see the over 800 domains we have available. You can also take a look at our general domain name guide to learn more about how domains work.