Link Farms are those websites which give backlinks to site without considering the niche.For example if someone has a clothes website and he is giving backlinks to automobile, real estate or other unrelated websites then that website is a link farm. Avoid taking links from such websites.
link farms are now days banned by google. and the links which are involved in the linkfarm as well. what ever the PR, the link should be genuine. Google counts with number of OBL on the particular site and checks for relevance then assigns the value. if your high pr site mate have many links means it's useless!
A link farm is a group of web pages that all links on every page in the other group. It is designed to increase the ranking of all sites in the results pages of search engines (SERPs). Search engines such as Google consider link farming as a form of spam and have implemented procedures to ban sites that participate in link farming.
A web page which is exclusively designed for trading hyperlinks and contains too many hyperlinks is considered as a weblink farming web page or in simple language web page which is selling hyperlinks is called weblink farm.
Link farm is a black hat seo.link farm means there is no relevant content,theme,but get number of back links to our site. link farming is to increase the number of sites that link to yours because search engines such as Google rank sites, the quality and quantity of sites that link to yours