Previously, Windows Phone's app switcher showed only an app panel (representative screenshot), with no way to close the app. As before, tapping on any of the app panels brings that app to the foreground (technically it triggers a fast resume of the gievn app).
The addition of the close button(s) tweaks the perception of Windows Phone's multi-tasking user experience. It gives users a greater degree of control over the apps that are shown in the app switcher. It's worth noting that, because of the way Windows Phone's system architecture works, there's no real benefit to closing an app from a memory or battery point of view.
Video description:
After installing the Windows Phone 8 Update 3 (GDR3) Windows Phone's app switcher (which appears with a press and hold of back button) now makes it easier to close apps. A close button (x) is shown in the top right of each app panel in the app switcher.
More on Windows Phone 8 Update 3 (GDR3)
We covered the key new features and updates added by Windows Phone 8 Update 3 (GDR3) in our original news story. The update will start being delivered over the air to existing devices next month. However, developers and interested consumers can get early access to the update through the Windows Phone Developer Preview Program.
In terms of the consumer roll out of the update HTC and Samsung devices are likely to get Windows Phone 8 Update 3 first. Nokia's Lumia devices will get the update a short time later, a result of Nokia's plan to include a number of its own updates and enhancements alongside the Microsoft update (i.e. as with the Lumia Amber update, Nokia will be delivering a significant firmware update on top of the Microsoft update).
See also
Video: Driving Mode in Windows Phone 8 Update 3
Video: Phone Storage in Windows Phone 8 Update 3