What is the difference between "external backlinks" & "referring domains" on Majestic SEO?

Externalbacklinks: Backlinks in general are links from other websites pointing towards your site. So that your site get traffic Google make your site top in serach results related your service.

Referral domains: Referral traffic is Google's method of reporting visits that came to your site from sources outside of its search engine. When someone clicks on a hyperlink to go to a new page on a different website, Analytics tracks the click as a referral visit to the second site.

An external link is the same as a backlink. It's a link that goes from an external website source to your website. But an internal link is when you link from one page of your website to another page of your website.

So a referring domain is where your backlinks are coming from, and backlinks are the links on the websites that link back to your site. Think of the referring domain as a phone number and backlinks as the number of times you've gotten a call from that particular number.