Doorway pages are webpages created specifically for high ranking in search engine results for particular keywords / queries. They serve as a direct point of entry for traffic originating from search engines and are not always made for being visited by visitors arriving through the home page.
Unlike doorway domains, doorway pages are not really sanctionned by search engines because generally they really respond to users’ needs and because it is often difficult to differentiate them from "natural" pages.
Some doorway pages are naturally created by faceted browsing functions, automatically created by use of templates and others have to be manually created. When made specifically (which is strictly speaking the "real" doorway page), doorway pages have often a text with high keyword density and on-page SEO is heavily used.
Some websites may have thousands of doorway pages automatically created from templates. Locations are often used for creating doorway pages and on ecommerce websites doorway pages are often used for covering queries matching product categories.
An example of doorway page automatically genereted by faceted browsing functions for the query "lace up booties":
An example of text with high keyword density at the bottom of a Gap doorway page for the query "turtleneck sweater":
An example of location based doorway pages on Booking.com :