A great post today from Matt Cutts who is finally back and posting again! Yeah. We missed you Matt. At least I did! It is no wonder you have no time in your life with all the spammy sites you must deal with on a daily basis!
Well anyway, the reminder was to avoid having doorway pages to your site. Matt was asked the question (and its a good question no doubt) about doorway pages and what they are, and its good to remind people that Google will not index you with these doorway pages. (or if you have snuck in somehow ... they will ban you eventually.) Trust me on that one!
So lets talk about what these doorway pages are (in my opinion-I am just one marketer). Typically, they are pretty full of keyword spam in the way they are created. You take one major keyword (phrase) and use it so many times on the pages and anchor it up to other pages on your site. Sometimes the doorway pages are created in other directories (folders on your site), or in a sub-domain. They can also be created offsite. All of the pages look identical (usually all doorway pages are template made) but switch out a keyword (phrase) ...
How do you know the difference between a good optimized page (going after certain keywords) and a doorway page? Provide GOOD content. Most of the doorway pages are identical except for the keyword (phrase) they are going after. If you want to show Googlebot you are the most relevant site (for the phrase), then create a page with good content and link up the right words to the right pages of your site. (AND SWITCH UP THE CONTENT!)
A good example on doorway pages can be found on Matt Cutts blog if you want to read more about this topic.