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.
There are two fundamental ways that the search engines use links:
Once search engines have crawled pages on the web, they can extract the content of those pages and add it to their indexes. In this way, they can decide if they feel a page is of sufficient quality to be ranked well for relevant keywords (Google created a short video to explain that process). When they are deciding this, the search engines do not just look at the content of the page; they also look at the number of links pointing to that page from external websites and the quality of those external websites. Generally speaking, the more high-quality websites that link to you, the more likely you are to rank well in search results.