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. Article directories are websites where users submit unique articles to be categorized and included to a specific niche. Well-written content articles released for free distribution have the potential of increasing the authoring business' credibility within its market as well as attracting new clients.