It is impractical to optimize all the pages of a large Web site for specific keywords and phrases. And some Webmasters may not want to draw in visitors with their secondary pages. Though search engines will index as many pages as possible, Webmasters want the visitors to arrive through the front page. This way people should get a better idea of everything a well-designed Web site has to offer. Some Web sites have dynamic pages which only exist when requested, so they cannot be indexed by search engines.
Doorway pages provide information about the sites' dynamic content.
Doorway pages attract visitors to content which can be described in more than one way. A doorway is simply an entrance to a larger site. Doorway pages have been abused. It is possible to create pages which fool search engines into indexing a site for the wrong phrases. Search engines discourage use of misleading content.