AKUs are Windows Mobile 5.0 updates that provide bug patches as well as new features. In many ways, you can think of them as point releases. Mobile-review.com has posted a review that takes a good look at each AKU that has been released and what it introduced or fixed for Windows Mobile 5.0. It covers all AKUs up to the recently released AKU 3.0.
It also has a handy tip to determine which AKU your device is running.
Source: Pocket PC Thoughts