Link Farm: A site carrying huge amount of link and no information. Few link directories are there which do not provide any information or they do not follow any submission guidelines as web directories do. They only meant for link submission or increasing subsequent PR value. Anyone can place any link at any time without any restrictions. These sites are known as link farm. Any search engine do not like exchanging links with them as they can not provide any relevant information neither they do help much in search engine optimization.
Hidden Links: Putting unnecessary and excess links to increase the link popularity for search engine, but links are invisible for normal visitors.