A 'link farm' is a page with an excessive number of hyperlinks.
Unless links are grouped into clear categories (that allow a user to focus on a specific section of the screen) this can result in 'navigation overload'. A user is forced to read every link to find the information they are seeking.
Link farms are often used to ensure a site is completely indexed by search engines. In this instance the 'farm will link to all or key pages within the site. A well constructed sitemap serves the same purpose.