Domain Mapping System allows to map domains to WP pages and/or custom types.
DMS allows one to map domains to WordPress pages, (custom) posts and (custom) archives.
Imagine you've got a company website based on wordpress with the domain http://www.mycompany.com, all good so far. Furthermore, you've got two products with own domains, e.g. http://www.myregularproduct.com and http://www.myspecialproduct.com.
Let's assume your product pages are micro-sites. One way to handle this, is by using WordPress Networks. One important note from WordPress is:
If you plan on creating sites that are strongly interconnected, that share data, or share users, then a multisite network might not be the best solution. (http://ow.ly/aQybY)
Well, if you need a shared blog or something similar, this Networks isn't what you're looking for. But DMS is.
With DMS you may point all three domains to the same Apache vhost (or nginx, lighthttpd, ...), where your wordpress instance is running on. You than create a WordPress Page (My Regular Product), perhaps with a custom template. In the DMS options you can now configure the domain http://www.myregularproduct.com to your WordPress page My Regular Product.
As easy as pie!
Note; the custom template is not handled by DMS, it's a built-in WordPress feature.