What is a Link Farm ?

1. A site that features links in no particular order which are totally unrelated to each other. Links on pages like these offer no link popularity benefit whatsoever.
2. A set of web pages that have been built for the sole purpose of increasing the number of incoming links to a web site. This is done in order to increase link popularity and search engine rankings. Can be considered spamming.
3. This is not a place where you want to be. In theory, you would submit your site to these places solely to increase your "popularity" (the links coming in to your site). Actually, if you are there then you can expect the reverse to happen. Search engines will likely penalize you -- maybe even ban your site -- as they consider this tactic to be spamming.
4. Also known as Black Hat SEO (Search Engine Optimisation)