By default, all four corners of the screen in Windows 8 and 8.1 trigger certain actions. Moving to the lower-left corner displays the Start button, while moving to the upper-left corner shows you the app switcher, with thumbnails for all open apps. And the upper-right and lower-right corners conjure up the Charms bar. If you ever find yourself accidentally triggering some of those hot corners, or simply prefer not to use them, you can easily disable on two of them.
From the Start screen, launch the Charms bar, click on the 'Settings' charm, then select 'Change PC settings'. From the PC settings screen, select the 'PC and Devices' option, then select 'Corners and Edges'. In the 'Corner navigation' section, turn off the 'When I point to the upper-right corner, show Search, Share, Start, Devices, and Settings' setting. Now, when you move to the upper-right corner of the screen, the Charms bar no longer pops up. The second setting is named 'When I click the upper-left corner, switch between my recent apps'. Turn that one off, and now you can move to the upper-left corner without triggering the app switcher.
Making these changes disables these hot corners, but it doesn't turn the features themselves off. You can still launch the Charms bar by hovering over the lower-right corner of the screen or by pressing 'Win+C'. And you can continue to display the app switcher by hovering over the Start button and moving up to the top of the screen.