Definition of link building:
Link building is the process of acquiring hyperlinks from other websites to your own. A hyperlink (usually just called a link) is a way for users to navigate between pages on the internet. Search engines use links to crawl the web; they will crawl the links between the individual pages on your website, and they will crawl the links between entire websites. There are many techniques for building links, and while they vary in difficulty, SEOs tend to agree that link building is one of the hardest parts of their jobs. Many SEOs spend the majority of their time trying to do it well. For that reason, if you can master the art of building high-quality links, it can truly put you ahead of both other SEOs and your competition.Why is link building important for SEO?
In order to understand the importance of link building, it's important to first understand the basics of how a link is created, how the search engines see links, and what they can interpret from them.Source:
Moz.com