"Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination.

Whether deployed across many domains or established within one domain, doorway pages tend to frustrate users, and are in violation of our webmaster guidelines."

http://www.google.com/support/webmasters/bin/answer.py?answer=66355

I've archived the original at my blog:
http://www.beussery.com/blog/index.php/2008/06/google-doorway-pages-new-definition/