In the early days of search engines, sites would add pages, usually called doorway pages, specifically to support a single keyword or phrase. Similarly, a doorway domain is a domain that is set aside specifically for improved optimization purposes. Spammers use doorway pages to rank their spam sites for a particular keyword phrase. A user would click the link on a search engine result page (SERP) and would then be presented with the doorway page. Clicking the link to go beyond this page would lead to a site totally unrelated to the search query. As search engines became more sophisticated, they caught on to this tactic and started ignoring and even penalizing doorway domains and doorway pages.
Generally, doorway domains should be avoided if at all possible. Often times, people will register a new domain name, optimize the index.htm or index.html file for a keyword, and then ask for people to visit their main domain. Doorways are usually pretty easy to find as the URLs tend to look like this:
Typically, each one of these domains would contain links back to www.maindomain.com with the links from the doorway domains accounting for a good portion of backlinks. Google has specifically stated that funneling links from a bunch of domains that you own to another main domain that you own is a spam tactic, and they will penalize your site if they find you using this tactic.
There are instances, and Google recognizes these, where you will want to buy all variations of your domain to keep spammers from knocking you down. If this is the case, you can set up 301 permanent redirects to the main domain. For an added layer of security, your robots.txt file should have "noindex" assigned to all the domain variations to keep them from getting indexed. The more variations of your domain name that you can register the better because it will prevent spammers from registering misspellings and hyphenated or abbreviated versions of your homepage to try to steal rankings.
Instead of trying to inflate your main site's PageRank and inbound link count with doorway pages, your time would be much better spent developing good content. If you can come up with unique, keyword-rich content for all of your domains and develop them into proper sites, even better! If there are insufficient resources to accomplish this, then 301s would be the way to go.