I run across a lot of questions about parking domains or redirecting them:

• If you have multiple domains, should you park them, or redirect them to your main site?
• What about creating "micro sites" on their own domains? Is that a bad practice?
  

I've read articles that have it completely backwards, recommending domain parking and staying away from microsites that direct traffic to your main site. In order to make sense of it, it's important to understand what Google is trying to avoid, which is duplicate content (AKA spam).

Scenario #1 - Using domain redirects rather than parking

Let's say a site has good content on one domain. Then the owner goes out and parks 30 more domains on it, with no redirects. That otherwise good content is watered down now, duplicated x30. Google's index is now bloated, and they feel (correctly) that they're being "gamed" with spam. Google's recommended practice is to redirect domains, not just park them.

Read this article about domain redirects from beginning to end and you'll get it.

Scenario #2 - Using micro sites

Let's say you have one main domain, but focused content that you want to put up on another domain directing traffic to your main site. There's nothing wrong with that, as long as the site is unique, the domain is unique, and the content is unique. Those are huge caveats, don't violate them.

Using micro sites is no different than another content site linking to yours. The only thing different is that the owner is the same. In a "free speech" world, you can have as many micro sites as you want if you have the time and resources to write them, and if they are not spam articles solely for the benefit of search engines. Then it's okay to link from them to your main site.

Whether they'll rank well and actually bring in significant traffic is debatable. What they can do, however, is add an inbound link to your site from a keyword-rich page. It doesn't have to have a high PageRank to help in that effort. Building micro sites is not something I would devote a lot of money to...yet I believe it can help in the whole SEO equation.

Think of it like Proctor and Gamble having a main site with a product page for Charmin, but also having a micro site for Charmin that deals in a more focused way with promoting that specific product. Both these sites point to each other. You could actually remove the link back from the main site and have it be a little more advantageous, since one way links are better than exchanges.