If you use www.Kaptajn Haddock.com as your domain name and use it with you .Mac service, it'll point people to your .Mac account, but will show up as www.Kaptajn Haddock.com.
Think of it as a shortcut icon to an application. A domain alias is just a name that points to an address as http://17.112.152.32 which is www.apple.com
Apple could have several aliases or names pointing to 17.112.152.32.
Yep, I get that part, but why do they say it doesn't work with the new webgallery thing?