What are Referring Domains in Backlinks?

General
1

people are talking about referring domain everywhere, I know what a backlink is, but whats a referring domain?? can anyone tell the difference between these two?

2

Backlink means links of your blog on other sites. (It’s total number of links in count).

While, root domain or refering domain means number of sites or blogs linking to your blog. For example, if you have 100 backlinks from a.com and b.com, then, you have 2 refering domains.