If you've ever accidentally tried to open a file you meant to install on your Windows Mobile device form your desktop, you have probably figured out that the ".cab" file extension isn't just a way to know it goes on a mobile device. It's actually a file archive, like a Zip file:
What's neat about that little factoid is that you can "unzip" that cab file on your desktop with WinZip and poke around inside. You can find the files that actually get copied over to your device. What's better is that, at least since Windows Mobile 5 landed on the stage, every cab file has something called "_setup.xml" inside it. _setup.xml is a set of instructions for your Windows Mobile phone - "Send this file to the start menu, make this change to the registry," etc.
It could potentially be very useful if you have installed a program that didn't seem to uninstall gracefully. Just re-download the cab for that nasty little app, take a peek at the _setup.xml file, then go into your device and manually clean up all the garbage it has left behind.
This archive file format has been re-purposed for application installation on Windows CE devices, by specifying a file with a special name that outlines the tasks required to install a given software package. This list of tasks will typically include decompressing and copying various files onto the PDA, but may also contain instructions to configure registry settings, or install shortcuts into the start menu etc.