Doorway Pages: Doorway pages are any webpages created as part of a spamdexing initiative to spam search engines. These pages are often designed to rank highly in search engines, only to redirect users to pages with advertisements or other content. They may also be referred to as gateway pages, entry pages, bridge pages, portal pages, and other names.
These sites also can pose a risk for internet users, as the pages sometimes contain malicious scripts and software, and others will redirect to websites with "spammy" or adult content. Typically, however, doorway pages are created to confuse search engines rather than to be clicked on by actual human beings.
However, some doorway pages are simply used to redirect users to content-rich sites and aren't meant to be seen. For instance, a website at www.example.com/index.php might use a redirection from www.example.com for its users. As long as this site is created in a search engine-friendly manner, without cloaking techniques, it shouldn't result in the site being banned from search results.