K.I.S.S. – Doors are things youre presented with prior to entering a home …
Doorway pages are presented to you prior to entering the primary page of a website.
Webmasters used to use this technique to artificially increase the relevancy of their web sites by using over saturated keyword rich pages as entries into their site. It was heavily abused and as such it’s treated as spam on most search engines today.
Undoubtedly some webmasters are still getting away with this to some extent as are they with cloaking, but I would caution you against employing any of the “cheating” techniques as its really not worth it for an already established site.