Link building was the first and most important criteria - it all started in the 90s when Google began to dominate the search engine market by using links as a classification factor. Google created an algorithm and tool called "pagerank", which was used to measure the quality of a page and it was mostly based on the amount of links pointing to the particular page.
At that time, they believed this is an optimal way to determine the quality of the page and its content, and very soon the overall classification algorithm started using almost only this website ranking system.
Today search engines use the links for 2 main purposes:
1) to find and index new web pages
2) to rank the sites in search results based on the links they have
When a search engine finds a link, it redirects to the linked page and retrieves its content and it uses then the network of links between the pages and retrieved keywords. These keywords are added to the search engines lists for the pages and in this way, they have enough information to classify a website against the relevant keywords. Search engines also evaluate the number of external links that lead to the site in question and the ranking of sites from these links. Generally, if a larger quantity of quality websites are connected to your website, the chance to get ahead in search results is much bigger.
So basically with just having a high-quality content, it's not possible to get to the top of the search results, but your also need to build links to your page and link building has a huge role in SEO.
In earlier periods, SEO specialists manipulated Google's measuring system by creating artificial or by buying thousands of links and pointing them to their pages. There were also many websites and services offering functionality of creating links or selling links for SEO purposes. These websites often called "link farm" (link farm) were however easy to detect and Google made improvements to exclude them or to lower their importance and even some had negative impact of the website ranking.
Google and its web experts on a daily basis try to create new improved algorithms and actively attempting to find websites that manipulate search results. They also make regular updates to monitor and discard pages that are not relevant to the rankings. In this way, the company breaks down many unethical practices, using updates such as Penguin and Panda. This is a serious blow to companies doing unfair and unethical link-building activity. Nowadays, the Internet is flooded with links and while some may be artificial or not relevant, there are still also many links that really define the quality of a website.
In the last years Google has been experimenting with the creation of page ranking methods that do not include links as a criteria including many other factors like the site loading speed or mobile-friendly tests and many others, but until today, Google has not managed to find an alternative way to classify pages that does not include the links as a basis. Google still keeps its classification algorithm secret, but as far as it is known, links are still a very important component of it.
There are many SEO techniques, and while each has its own advantages, search engine optimizers agree that link building is one of the most challenging tasks when working on the site SEO. So if you want to go a step further ahead of your competition and have better website ranking, always consider the link building and try to have as many links from quality websites as possible. Pay attention when linking between your own sites to not link between sites registered on one Google Analytics profile (since Google will know both sites are owned by your) and also to not link between sites of yours hosted on the same IP (since again Google could easily understand the sites are owned by the same person and trying to spam with links and penalize the site) and avoid having 2 pages linking one to the other (A linking to B and B linking to A).
