What are Doorway Pages?
Doorway pages are pages that have been generated to rank highly in a specific keyword or phrase.
If a Webmaster wanted to ensure that a particular part of his/her Web site was visited they could create doorway pages with keywords/phrases that would definitely be entered into a search engine. Thus standing a better chance of getting listed in the top 10 positions.
Doorway pages are not a main entry to your website, normally you would have a 'click here' or some JavaScript to redirect the visitor to your main site. You have probably seen many doorways while surfing without realising what they were for.

What makes a Doorway page?
A doorway page contains all of the elements associated with any other Web page. The only difference is that the title, keywords, description, and body content have all been optimized for one specific keyword or keyword phrase.
However, you should be very cautious in using tables, images or heavy JavaScript as these can unbalance your carefully optimized Web page.

Are Doorway Pages allowed?
The simple answer is yes, but as with everything in Web promotion you need to be careful that you are not spamming the search engines. 
Keep it simple, if you create 12 doorway pages to your main index page then this is not deemed acceptable by the engines. However if you have a large website and want to advertise individual elements of it, that would normally be impossible to do with Meta Tags then this is allowed and can help you get higher positioning.
The simple rule to remember when adding doorway pages to your site is that your site could be checked by hand if flagged as a spammer. If this happens you want the doorway page to look like a part of your site whilst keeping the keyword optimisation intact.

Should I create a Doorway Page?
Okay, lets have an example of a website that sells computer equipment.
With Meta Tags you will only be able to inform a potential visitor of a few products that you are selling (if any), however with doorway pages you could optimize a doorway page for each type of product. This way all your products will be listed with the engines thus making your website receive more traffic.

Doorway Page Maker