Microsoft changed the way aliases are managed on Microsoft accounts. Bottom line is that your Microsoft account can have several aliases--other email-address-like identifiers--as the user name.
The aliases can be Microsoft-type email addresses (Live, Hotmail, Outlook), organizational email addresses, or even third-party aliases (yes: your gmail address!).
You can sign onto Microsoft services and devices with any of your aliases. Regardless of which you use, you are actually the same underlying account. That means settings, data, and purchases (apps, games, music) are available.
You can identify any one of these aliases as your "primary" alias. It seems the main "role" of the primary alias is now, simply, a display-related role. It shows up on your login screen and on the background of your XBox. Things like that.
It's a huge step in the right direction. Microsoft calls it "simplifying" and that's pretty close to accurate. You can read up on the details in the Microsoft announcement.
Windows Phone Alias Caution
But caution if you're a Windows Phone user!!! Read Paul Thurrott's article on the problem with changing your primary alias.
I can guess and hope that this direction will extend with persona-related functionality, so if I log on to my device as my "personal" alias, I get a certain experience, or my "business" alias, another, but my underlying account is the same.
Perhaps with claims-based authentication, my access to resources, configuration, and functionality could be based not just on who I am, but what device I'm on (already well underway with current Microsoft stacks) and my "persona." That would be cool!