Link building is an important part of SEO that involves increasing the number and quality of inbound links to improve search engine ranking. There are different types of links such as editorial links from high quality sites, resource links on curated pages, and acquired links obtained through paid methods. Links can also come from social bookmarking, blog comments, directories and image links. White hat link building follows search engine guidelines to add value for users, while black hat techniques are against policies and can risk site bans.