Some directories are very general in scope and list websites across a wide range of categories, regions and languages. Many concentrate on restricted regions and single languages and some are restricted to specialist fields. RSS directories are similar to web directories, but contain collections of RSS feeds, instead of links to websites.

Examples of well known, general, web directories are Yahoo! Directory and the Open Directory Project (ODP). ODP is significant due to its extensive categorization and large number of listings and its free availability for use by other directories and search engines.

A human-edited directory is created and maintained by editors who add links based on the policies particular to that directory.

Human-edited directories are often targeted by SEOs on the basis that links from reputable sources will improve rankings in the major search engines. Some directories may prevent search engines from rating a displayed link by using redirects, nofollow attributes, or other techniques.

Another direction taken by web directories is the paid for inclusion model. This method enables the directory to offer timely inclusion for submissions and generally fewer listings as a result of the paid model. They often offer additional listing options to further enhance listings, including features listings and additional links to inner pages of the listed website. These options typically have an additional fee associated, but offer significant help and visibility to sites and/or their inside pages.

Directories have various types of listings, often dependant upon the price paid for inclusion:

• Free Submission - there is no charge for review of the site
• Reciprocal Link - the site submitted must link back to the directory in order to be listed
• Paid Submissions - a fee is charged for reviewing the submitted link
• No Follow - there is a rel="nofollow" attribute associated with the link, meaning search engines will not follow the link.
• Featured Link - the link is given a premium position in the category where it is submitted
• Featured Homepage Link - the link may be listed on the homepage of the directory.

Reference: Wikipedia
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