If you use the Mac App Store and want to switch from one section to another – from Featured to Updates, or from Purchases to Top Charts, you can do this by clicking the icons in the app’s toolbar.
But you can also do this from the keyboard. Press Command-1 to go to Featured, Command-2 to go to Top Charts, and so on. Like many windows in OS X, these keyboard shortcuts let you switch tabs. It’s not a big deal, but it’s a bit quicker than mousing.