Doorway pages are pages that have been designed for one purpose only, to rank high in the search engines for a particular keyword and then direct the arriving visitors further into the site. Usually, their design is relatively simplistic and includes a lot of text, but contains very little graphics or other fancy effects.
Doorway pages focus on particular keywords and phrases targeted for search engines and serve as an entry page to visitors. These used to be a popular means to achieve high rankings with Search Engines, but recently, many Search Engines have started completely ignoring websites with doorway pages.
Because creating doorway pages used to be so easy, especially when using programs that can automatically generate hundreds or thousands of pages in a heartbeat by using a simple template and merely placing the keywords in the right places, many sites lost control and created enormous amounts of them.
The search engines initially tolerated this behavior, but as their databases started being filled with doorway pages, they soon realized that these pages would quickly conquer the top ranking spots from standard content-rich pages. It would reduce the quality of their indices and lower the value of the search results, resulting in users abandoning the search engine.
Search Engines such as AltaVista began removing doorway pages from its index and in some cases even banning the sites that were using them. Other engines followed suit, and soon it was safe to say that the age of doorway pages, at least in the form we used to know them, had passed.
Instead of focusing on doorway pages, you should focus on optimizing your actual content pages.