A doorway page is normally one page highly optimised for a keyword with a prominant link to the main website. This makes ranking for a particular keyword much easier as you do not have the normal restraints associated with optimising a working website. The technique, however, is not looked upon favourably by Google because it allows webmasters to direct visitors to a non relevant page. Using such a technique is done at your own risk, Google is said to have algorithms for detecting doorway pages and will impose penalties for using them.
Doorway pages - the lesser you know the better it is; classified as black hat and would attract severe penalties. If you would know less about it you would not be tempted to use it and thus avoid the wrath of SEO agencies. It is a easy way out but frowned upon.