If you buy an app from the Windows Store, you can make it available to other users on the same Windows PC. This is useful if you, say, buy an app for your child to use. Here's how you do it. (This is all explained on the Windows Store blog, but I've converted it into a step-by-step and updated it for Windows 8.1.)
First, sign on as yourself and install the app under your own account.
Next, sign on as the child (or whatever other account you want to share the app with), and launch the Store from that second account.
In the Store app, go to the top of the screen and hit Account, then My account.
From the My account page, use the Change User button to sign out as the child account and sign in as yourself.
Once signed in as yourself, you can reinstall the app into the child account. You can do this the hard way, by searching for the app, or the easy way by hitting Account at the top of the screen, and then choosing My Apps. Tap the app you want to reinstall, then hit the Reinstall button. (Since the app is already installed, all this does is increment the reference count on the app.)
When finished, sign out of the Store from the child account.
In Windows 8, each purchased app could be used on up to five PCs, regardless of how many times it was installed on each PC, so adding an app to a second account did not eat into your device quota. In Windows 8.1, the limit was bumped to 81 PCs, which means that for most people, the device limit will not be problem.