Backlinks are links from other sites. Think of them as votes of affirmation. One vote can come from a domain so for SEO purposes it doesn’t matter if there are 100 or 1 link from the same domain one link is the count you gain. Now those other links may increase traffic to your site but in regards to SEO its one vote.
The more domains that link to you the more authoritative you must be right? Well Kinda. If 1000 domains link to your site you likely are more authoritative than a site that 3 sites link too. Not all domains or votes or backlinks….are the same. A link to your site from UltimateSEO.org carries with it the weigh attributed to that site by its backlinks. Different companies refer to the authority of a site differently.
You can see a site’s backlinks from many indexes most being paid. Ultimate SEO recommends Monitor Backlinks if you want a tool that is really good at backlinks. UltimateSEO received nothing for that endorsement. The endorsement or vote …. as you see is a backlink.