A backlink the word used to describe an inbound hyperlink from another website.
The number, quality and diversity of anchor text and referral domain are factors used by Google to define the quality and importance of a domain or webpage. Backlinks, to this day, are very valuable for SEO. The value of the total inbound backlinks to any given site is then used to determine how far up a website should rank in the Google SERP.
In the past, website owners could buy backlinks, create blog networks or use other blackhat link building techniques to artificially inflate their link profiles. However, after the Google Penguin update, websites are punished for these activities.