The link provided by takemyoxygen already gives a lot of technical details. I'd just like to add a couple of additional notes.
Not all versions of Windows 8 have the sideloading license for installing Windows Store apps outside the store:
- With Windows 8 Enterprise machines joined to a domain you're ready to go.
- With Windows 8 Pro (domain joined or not) and Windows 8 Enterprise outside a domain you need to buy a special activation key (sold in batches of 100).
- With Windows 8 there's no way to get a sideloading license.
- With Windows RT (ARM) you need the activation key. The machine can't be joined to a domain.
Once you have a sideloading license, you can install a Windows Store app package:
- You can manually run the installation script creted by Visual Studio along with the package.
- You can set up a group policy if you have Microsoft System Center and the machines are domain joined.
- For Windows RT machines you can use Windows Intune (as a standalone service or Microsoft System Center add on) to create an internal store for installing Windows Store apps.
Make sure you visit Lhotka's blog to read more on the sideloading subject.