Link building is the process of getting other sites to link to your site in order to increase its popularity and search engine rankings. There are two main types of links - reciprocal links, where sites link to each other, and one-way links, where a site links to yours but you do not link back. Some link building practices to avoid are using noindex or nofollow meta tags, JavaScript or redirected links, and blocking search engines from indexing link pages via a robots.txt file in order to artificially inflate a site's link popularity.