Windows Mobile 5.0 is the operating system used by a huge variety of amazing pieces of hardware, but it's not fixed in stone. Every time a new piece of hardware appears on the market, perhaps with a new screen size or a new keyboard, the operating system needs a little tweak. These tweaks are called "AKUs" or "Adaptation Kit Updates".
Should you care? Most of the time, no, as a developer you don't need to care. As an End User you don't need to care either. Remember: the goal of Windows Mobile is primarily to create a consistant platform that will always run your applications. If you compiled it for Windows Mobile 5, it should run on any Windows Mobile 5 device (and if you have taken the time to make your application is screen-size and orientation aware, chances are that's exactly what will happen).
There are exceptions of course: for example, the release of the Messaging and Security Feature Pack. This was an update that provided some cool new email features. This was included in AKU2, and some device manufacturers / mobility operators provided updates. I've just updated my Palm Treo for example. So, potentially, some hardware out there has MSFP installed, and some doesn't.
In case you need to programmatically determine which device has which AKU installed, there is now a page on the Windows Mobile developer Wiki that lists the various AKU releases, and how to work out which one is running on your hardware.