Link farms and web directories.
There are a number of key differences between link farms and web directories. The most prominent difference between the two is that web directories are valued by the search engines, while link farms are considered to be spammy. Link farms are generally built by webmasters who wish to cheat the search engines. They try to jam a bunch of links on a page with the hope that it will rank well. In most cases, it doesn't, and the search engines may ban websites that feature this. Web directories are much more legitimate. They are designed to help people find what they are looking for on the web.
A web directory could be likened to the traditional yellow phone book directory. With a web directory, you have a bunch of websites that are placed in various categories. When you click on the category page, you will have a list of websites. Each website will have a title that comes in the form of a link, and they will all have descriptions which tell you what the site is about. By reading these descriptions, you will be able to determine whether or not you're interested in visiting the website. Link farms are the exact opposite. They tend to have a bunch of links on a page that don't have any descriptions. If you use the Internet frequently, it is likely that you've come across these websites.
They are easy to spot because they don't have any information that is useful. They are generally a bunch of random links on a page that will take you to different places on the web. Because search engines are interested in helping humans find what they are looking for on the web, it is easy to see why they look down on link farms. If a search engine has too many link farms in its index, people will become annoyed when trying to find useful information, and they will eventually begin looking for other search engines to use. Many blackhat SEO people use link farms in an attempt to get to the top of the search engines.
While this is great for them, it is not so great for the search engines. And this is precisely why search engines ban or penalize sites that have link farms. It is bad for their business, and it can hinder them from being successful. Web directories can be thought of as mini search engines. The main thing that separates the two is that many web directories will be edited by humans rather than an advanced algorithm or neural network. Web directories assist search engines because they make it easy for them to to crawl websites and add them to the index.
