Doorway pages are meant for spamming the index of search engines.
Usually, when a visitor clicks on a doorway page listed on SERP, they are redirected with a meta refresh command or javascript to another page. Doorway pages are also redirected using a server side redirect like a .htaccess file or server configuration file.
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Doorway pages serves two purposes. One, more pages are indexed on search engines and the second, they get more visitors land on their websites.
This issue has been identified by many search engines and they do penalize websites who use meta refresh command on their doorway pages.
Cloaking and Doorway Pages
Cloaking is also a form of doorway pages where two different versions of the page is presented. One version to visitors and another to search engine crawlers using server side scripts. Using a user-agent or IP address the cloaking website figures out if the call is from a human or from an automated crawler.