The Doorway is a secondary web page optimized for particular search queries and intended to attract a visitor at a Web page only. Such a traffic is usually redirected to a customer's or a doorway owner's primary site by redirecting or with help of the primary site advertisement.
As a rule, doorway is created with help of special generator programs and is saturated by keywords at the expense of the basic meaning. The doorway page itself doesn't represent interest for a user, the search services will read it off as the spammers pages which are clogging search engine issue, that is why the services are excepting them or reducing their index.