Modern UI apps are what Microsoft calls Immersive applications, which basically means they run full-screen, but there are ways to view up to four at once. On Windows 8, swipe from the left and the last app you were using will turn into a thumbnail. Drop this, and one app displays in a sidebar pane while your current app takes the rest of the screen. And you can then swap these by swiping again.
Windows 8.1 expands on this and can display up to four apps simultaneously, if you have enough screen space. Move your mouse cursor to the top of the screen, and when it changes to a hand icon, drag and drop your app to the left or right. Once you've moved it enough, a dividing line will appear. You can drop the app, and it'll appear in just that part of the screen. Use the bar between your apps to resize their window widths, or if you need to make one full-screen again.
