New Start Screen
With Windows 8.1, there are now four different tile sizes on the Start screen; Huge, tiny, wide and medium.
All Apps
When you install an app, it won't automatically populate the Start screen with another tile. It will go on the All Apps page, which you can now get to by swiping down on the Start screen.
Search
"Search Heroes" are Microsoft's term for these deep, rich result pages you get when you search for a popular performer or a city. Here, searching "Chicago" brings up a map, current, weather, and attractions.
Search Hero
Searching on a pop star results in a page that lets you play his top songs, as well as showing web information.
Internet Explorer 11
With Windows 8.1 comes a new browser version, Internet Explorer 11, which supports far more web technologies (including WebGL) and offers best-in-class touch input capabilities. It also now lets you keep the tab and address bar always showing in its new-style, full screen version.
Radio
New for Xbox Music is a Pandora-like Radio feature. It's ad supported unless you get a subscription.
Split Screen
No longer are you limited to a thin sidebar or even just two windows with the new-style interface. Depending on developer choices, you can adjust windows to any proportions, and have up to four on a screen, depending on your display resolution.
Add Window
Here's how it looks when you add a third window to a split screen view.
Cropping in Photos
The included Photos app gets really useful in Windows 8.1, with editing and enhancement capabilities.
Settings
With Windows 8.1, the modern interface (formerly "Metro") gets far more settings, saving you a trip to the more complex Control Panel on the Desktop.
Store
The Windows Store gets a redesign with Windows 8.1. You can see the description, screenshots, and ratings all at once, without tabs.
Skydrive, Soon to Be OneDrive
SkyDrive (in the process of being renamed to OneDrive) can now sync your documents without the need to install a separate desktop client.