What is a Link farm?
A Link farm is a group of websites specially designed for raising a number of outbound links to other websites. So, the another website's popularity and indexing are raising through a link farm. Hyperlinks are the biggest part of the content of the link farm, which seems like a normal web page. The link farm can be made manually or through automated programs.
Although it brings some benefits, the link farm is considered as negative and it is a type of search engine spamming.