SEO FAQ - Search Engine optimization

Link farms are links pages that are solely build for the purpose of building links for search engines, Usually a link farm will contain links to lots of unrelated pages from a links page.

But directories have relevant categories and they link out to sites only from relevant categories, Directories provide value to visitors as well as search engines, Link farms are solely build for search engine purposes, if search engines don't exist link farms wont exist

Also there are sites doing excessive cross linking even those sites are called link farms since the cross link to cheat the search engines for link popularity.