Link firms are the places where you can find lots of link in same domain of other sites these are really bad in SEO. Always be away from this type of site it may penalize your site in Google.
Links farms is a certain web page or site that contains any sort of links from different niche sites. The list is not arranged in order and mostly, the links came from spam sites related to porn, medicines, gambling, etc.
On the World Wide Web, a link farm is any group of web sites that all hyperlink to every other site in the group. Although some link farms can be created by hand, most are created through automated programs and services.
Link Farm is nothing but a webpage that contains bulk of unrelated links. It is one of the Black Hat SEO techniques and I strongly suggest not to use it because as a result of using this technique, there are chances that your site may get penalized or banned by Search Engine.