Link farm or link spams are those web pages which stuff excessive keywords in their content to get backlinks and this is one of the Black Hat SEO technique. However, search engines track out such spam pages and ban them from their search results. Hence it is inappropriate to submit your link on link farms.
Link farming is the process of exchanging reciprocal links with web sites in order to increase search engine optimization. It is considered as black hat method of search engine optimization(SEO) because the content on the web sites linking to each other is often not related and is often of a very low quality.
A link farm is a group of websites or a mere website created only for the purpose of increasing the link popularity of another site by increasing the number of incoming links.
Basically, it's to increase Google PageRank. A website or a group of them having a large number of outgoing links to other websites. You have to be very careful with it though because Google and other major search engines don't accept this unnatural way sometimes.
It's the process of exchanging reciprocal links with Web sites in order to increase search engine optimization. Google consider link farming as a form of spam and have been implementing procedures to banish sites that participate in link farming, so the term link farming has garnered negative connotations across the Internet.
You need to be very careful about this link acquiring strategy. A lot has changed since Google Penguin update.