Backlinks are incoming href's or hyperlinks to a website or web page. Inbound links were generally considered important (prior to the invention of Bing, Yahoo and  Google) as a the only method of navigation among websites; today they are significant in search engine optimization (SEO). The total count of backlinks is  a basic  representation of the popularity or "weight" of that site or homepage (other measures, although these days being slowly phased out, such as PageRank, are likely to be more relevant). Outside of SEO and Linkbuilding, the backlinks of a webpage may be of significant personal, cultural or semantic interest: they indicate who is visiting and reading that page.

In basic link vocabulary, a backlink is any hyperlink received by a web address (web page, message board, web-site, or top domain) from another web node. Backlinks are also called forward links, inbound links, inlinks, and inward links. One Referencing the other.

Search engine rankings:

Search platforms often use the amount of backlinks that a site has  pointing to them as a very important factors for calculating that website's ssearch engine results page ranking. Google's run-down of the PageRank system, notes that "a link from page A to page B as a vote, by page A, for page B.^" This information of the workings of search engine rankings has driven a large portion of the SEO industry into methods commonly termed linkspam, or attempting to place many inbound links as possible to their page despite the context of the original site.

Most Pages likely employ multiple methods (known as search engine optimization, usually just SEO) to increase the magnitude of backlinks pointing to their website. Most methods are free for use by everyone. Others ike linkbaiting require a lot of planning and coordination to be successful. Some fall on "linkbaiting" accidentally. The pages with 'breaking news' about a celebrity are good examples. After setting up some good "linkbait", many websites will link to the site because information there is of great interest to a lot of people.

There are many areas that determine the "weight" of a backlink. Links from authoritative sites on a specific  topic are greatly valuable. If both sites have similar topics, the backlink is assumed relevant and has strong influence on the rankings of the webpage for that keyword. A backlink represents a 'positive vote' for the pointed-to webpage from the other granting webpage. Other important factors are the anchor text of the link and the top level domain type, be it .com, .edu, .mil, .gov , etc... Anchor text is the label of the link as it appears on the site, these links are clickable. Search Spiders and crawlers sort through the anchor text and page topic to evaluate the relevancy to the content on the webpage. Webpage link and content congruency are importantly weighted in engine results page (SERP) listings of your page with respect to the keyword query by a user of the search engine.