Windows 8 is perfect for multitasking, especially thanks to the new snap feature for apps. To use snapping, you'll first need to have one app open and another one running in the background. Note that the Desktop counts as an app.
Using a touchscreen, swipe your finger in from the left edge of the display to open a list of running apps. Drag the app you wish to snap from the bar to the left or right edge of the screen and drop it when an open space appears. The app should snap into place, and now you can choose which app is larger by dragging the black bar to the left or right.
If you're using a mouse, you'll need to move the cursor to the top left corner of the screen and move it down the edge of the screen until the app list appears. Simply drag and snap the app in the same manner as the touchscreen.