A web directory is a directory on the World Wide Web. It specializes in linking to other web sites and categorizing those links. 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 one or two categories. Web directories often allow site owners to directly 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.