The initial display is very like Alt+Tab which allows task switching, but also has an Add a desktop option:
The Add a desktop option allows for the creation of a virtual desktop. This is the first version of windows which allows multiple desktops to be created, although this functionality has existed in add-ons since at least Windows 95.
The screenshot below shows the second desktop with only IE open; the one above shows the first with several applications open:
Whilst Alt+Tab task switching shows all open windows from all desktops, Win+Tab shows the tabs open on the current desktop while also allowing switching between desktops using the second set of preview tiles at the bottom of the screen:
|Windows 10 Technical Preview|
|The Start Menu|
|Start Menu Customisation|
|Win+Tab and Virtual Desktops|