With a Windows Live ID (also called Microsoft account), you can access all Windows Live and Zune services with a single username and password on your computer or phone. You can also use the same username and password to access Xbox Live services on your Xbox.
With your Windows Live ID, you can, for example:
Download content from Marketplace
Back up your contacts in Windows Live
Upload, store, and share pictures and documents on SkyDrive
Get your gaming achievements on your phone, and add to them when you play games on your phone
Keep track of and lock your lost phone with Find My Phone
To learn more about Windows Live ID and Windows Live services, go to http://www.live.com/.