Link building is one of the key factors for generating traffic and increase SERP on search engines. In link building commonly includes two types of activities i.e., off page SEO activity and on page SEO activity. In off page SEO, you have to do - directory submission, social bookmarking, forum posting, blog commenting, article submission etc., a lot of most other activity. And on the other hand you have to do proper on page activity like- title tag and description tag creation, proper keyword selection, internal linking, footer linking. Sitemap creation and submission etc., a lot more.
But you have to take care while link building i.e, try to build relevant links for avoiding any penalty from search engines.