Although Windows 8 Apps are designed to be run full screen - they can also be snapped to the left or the right of the screen so that you can have more than one app visible at a time - and you can easily adjust them by dragging the black handle (as circled below)

 

Depending on your screen resolution (minimum is 1024x768) you can snap them in 50:50, 40:60 and 30:70 configurations - on higher resolution screens of 1920×1080 and above you can split between three apps at one time

 

Apps can be snapped to the left or right of the screen by dragging them down from the top and then to the left or right, or by using the following keyboard command:-

 

 

The example below shows the Weather app snapped to the right hand side of the screen whilst you continue to use the desktop to the left