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A web directory is not a search engine and does not display lists of web pages based on keywords; instead, it lists web sites by category and subcategory. Most web directory entries are also not found by web crawlers but by humans. The categorization is usually based on the whole web site rather than one page or a set of keywords, and sites are often limited to inclusion in only a few categories. Web directories often allow site owners to submit their site for inclusion, and have editors review submissions for fitness.
RSS directories are similar to web directories, but contain collections of RSS feeds, instead of links to web sites.
Scope of listing 
Most of the directories are general in scope and list websites across a wide range of categories, regions and languages. But some niche directories focus on restricted regions, single languages, or specialist sectors. One type of niche directory with a large number of sites in existence is the shopping directory. Shopping directories specialize in the listing of retail e-commerce sites.
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.
However, a debate over the quality of directories and databases still 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, such as using automated submission of related links by 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 features in listing, often depend upon the price paid for inclusion:
- Free submission – there is no charge for the review and listing of the site
- Reciprocal link – a link back to the directory must be added somewhere on the submitted site in order to get listed in the directory
- No Reciprocal link — a web directory where you will submit your links for free and no need to add link back to your website.
- Paid submission – a one-time or recurring fee is charged for reviewing/listing the submitted link
- No follow – there is a
rel="nofollow"attribute associated with the link, meaning search engines will give no weight to the link.
- Featured listing – the link is given a premium position in a category (or multiple categories) or other sections of the directory, such as the homepage. Sometimes called sponsored listing.
- Bid for position – where sites are ordered based on bids
- Affiliate links – where the directory earns commission for referred customers from the listed websites
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, Salehoo and World Wide Web Virtual Library, are edited by volunteers, who are often experts in particular categories. These directories are sometimes criticized due to long delays in approving submissions, or for rigid organizational structures and disputes among volunteer editors.
In response to these criticisms, some volunteer-edited directories have adopted wiki technology, to allow broader community participation in editing the directory (at the risk of introducing lower-quality, less objective entries).
Another direction taken by some 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 web site. These options typically have an additional fee associated, but offer significant help and visibility to sites and/or their inside pages.
Today submission of websites to web directories is considered a common SEO (search engine optimization) technique to get back-links for the submitted web site. One distinctive feature of 'directory submission' is that it cannot be fully automated like search engine submissions. Manual directory submission is a tedious and time consuming job and is often outsourced by the webmasters.
Bid for Position directories 
Bid for Position directories or also known as bidding web directories, are paid-for-inclusion web directories where the listings of websites in the directory are ordered according to their bid amount. They are special in that the more a person pays, the higher up the list of websites in the directory they go. With the higher listing, the website becomes more visible and increases the chances that visitors who browse the directory will click on the listing.