General

Some directories are very general in scope and list websites across a wide range of categories, regions and languages. But there is a wide range, and many focus on restricted regions and single languages. Some are restricted to specialist fields.

Examples of well known, general, web directories are Yahoo! Directory and the Open Directory Project (DMOZ). DMOZ is significant due to its extensive categorization and large number of listings and its free availability for use by other directories and search engines (many sites violate its terms of use by using its content without acknowledgement).

A debate over the quality of directories and databases continues, as search engines use ODP's content without real integration, and some experiment using clustering. There have been many attempts to make directory development easier, whether using a "links for all" type link submission site using a script, or any number of available PHP portals and programs. Recently, social software techniques have spawned new efforts of categorization, with Amazon.com adding tagging to their product 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, just like www.999pro.com

* 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.

[edit] Human-edited directories

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.

Many human-edited directories, including the Open Directory Project and the World Wide Web Virtual Library, are edited by volunteers, who are often experts in particular categories. These directories are sometimes criticized [1] due to long delays in approving submissions, or for rigid organizational structures and disputes among volunteer editors.