How is this different than Adobe Flex?
In November, 2011, Adobe announced they were no longer supporting Adobe Flex. Instead of just killing the framework they donated it to Apache. Apache Flex 4.8.0 is essentially the exact same version of Flex as Adobe's last official version. Since that initial donation the Apache Flex team has been working hard on fixing bugs, providing additional components and finishing unfinished components. As we progress through additional versions, we will be less and less like Adobe's Flex framework, but we will strive to be compatible with it.
Adobe has offered some of their customers extended support contracts, and may produce additional versions, but that is unlikely. Future support of the Flex Framework is through this project.
There are a few differences you should be aware of in the Apache version of the SDK. Most notably, the Flash Player will not cache RSLs created with Apache Flex. You can find out more in the RELEASE_NOTES file in your SDK download