In Windows 8, you can now optionally sign into Windows with a Microsoft account using your email address. This account stores many of your personal Windows settings, preferences, and saved passwords, as well as other items like browser history, favorites, and Windows 8 apps, on Microsoft's servers.
Whenever you log on to a new Windows 8 device with that Microsoft account, all your data automatically syncs to your new hardware.
Although this new syncing functionality can be useful, it does pose a security risk.
Read on, some good advice - article at PC World.