A backlink is a link that points to a given website or webpage from another website. A backlink is more often a textlink, but it can also be associated with an image or be embedded in a Flash file.
Backlinks are very important to promote a website and to gain traffic. There are a direct source of traffic (individuals who click on backlinks land on the website) and above all, there are a means (with on-page elements) to optimize SEO and to obtain good ranking on SERPs.
Indeed, search engines consider backlinks as votes in favor of the linked website. Originally, the more backlincks a website had, the best ranking it had. But SEO specialists, and particularly black hat SEO experts, have rapidly found many ways of developping thousands of backlinks through linking strategy. The most part of these backlinks are inorganic or unnatural backlinks.
To adress this phenomenom, Google and other search engines have developed algorithms to take into account the quality of backlinks for ranking a website.
When analyzing backlinks Google take into account:
backlink placements in the webpage
anchor text of backlinks
follow or no follow commands
quality and trust associated with the website which display backlinks
hints of unnatural backlinks ( paid backlinks, reciprocal links, spinned content, etc.)
Backlinks are a cornerstone of good search engine ranking.