Doorway pages is a part of Black hat SEO. Doorway pages are simple HTML pages that are customized to a few particular keywords or phrases, and they are programmed to be visible only by specific search engines and their spiders. The purpose of these doorway pages is to trick the search engines into giving these sites higher rankings; this sounds okay until you realize that they are not static destinations.
Doorway pages are easy to recognize in that they have been designed mainly for search engines, not for human beings. This page explains how these pages are delivered technically, and some of the problems they pose.
Doorway pages are web pages that are created for spamdexing. This is for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page.
Doorway Pages are used for Spamming of Search engine index and get high rankings in SERPs but it is a black hat techniques. These type of pages are especially created for crawlers for getting high rankings and shows different content to users.