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. 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 directly submit their site for inclusion, and have editors review submissions for fitness. Most of the web directories are very general in scope and list websites across a wide range of categories, regions and languages. But there are also some niche web directories which focus on restricted regions, single languages, or specialist sectors.