Mobile apps and computer software often have different interfaces. In the traditional desktop environment, it's possible to multitask and run several programs at once. Each program runs within its own window. You can resize windows and have several programs in view at the same time.
The OS handles multitasking by managing and directing resources to each running application. The more programs you activate at once, the more work your computer has to do to keep them running. And if you're using several processor or memory-intensive programs at once, you may find progress slowing to a crawl.
In general, modern desktop and laptop computers can handle a reasonable load of multitasking because they tend to have enough memory and processor power for the job. But the apps that run on mobile devices are usually more limited. They may not have as much memory or horsepower as a PC to dedicate to multiple running processes.
Apps tend to take up an entire screen's worth of landscape. Most operating systems that run apps only allow you to view one active app at a time, even if the OS supports multiple running apps. You may be able to switch from one app to another seamlessly, but you can't view more than one at any given moment.
The tiled interface for Windows 8 works more like the second method -- the running app takes center stage. But you can make it act a little like the desktop environment with a feature Microsoft calls Snap. Snap lets you assign an app running in the background to either the left or right side of the screen while the app you're running in the foreground takes up most of the screen's real estate.
Microsoft also lets you Snap your desktop to the side of the screen. This will let you see all applications your computer is currently running. It's like the application tabs in older versions of the operating system.
The Snap version of an app may have only a few features -- such as notifications -- enabled. You'll be able to see both apps at the same time, but you may not have access to each app's full functionality the way you would with software windows in desktop mode.