A link is a connection between web pages or sites. Quality backlinks can increase a site's authority, page rank, search rankings, and traffic. Link analysis tools like Google Webmaster Tools and Ahrefs help analyze links. Links can be one-way, with one site linking out but no link back, or two-way/reciprocal with sites linking to each other. Multiple sites can share three-way links. DoFollow links pass PageRank while NoFollow links do not. Effective link building focuses on quality over quantity, diversifies link sources, and avoids exact match anchor texts. Methods of link building include guest blogging, forum posting, and social bookmarking.