It replaces the Windows Update dialog with an Windows 10 advertisement and it adds a second start button to the startmenu that lauches an advertisement app. If you click the wrong button it will automatically download Windows 10 in the background - Microsoft's way to not repeat the Vista, Win8 and Win8.1 launch fiascos. To each their own, some also like the Ask toolbar that comes with Flash installer.
http://www.infoworld.com/article/2907472/operating-systems/w...