I spent 7 years at a large e-Finance company working on an Application Server for Windows with a team of very smart folks. When we'd go and sell our application server/component container to banks, we'd have to say things like "Windows doesn't really have an actual App Server like jBoss or WebSphere, so we wrote our own." However, remember that we were in banking, not in appserver-writing, so I always thought this was cheesy. As Microsoft came out with different subsystems that did stuff we'd already done, we'd evaluate them and "refactor via subtraction," removing our stuff and moving over to the MS stuff when appropriate. Still, the lack of an AppServer was an irritant.
AppFabric is the Windows Application Server. For web applications, AppFabric gets you caching (remember "Velocity?") for scale as well as high-availability of in-memory data. That means replicated, in-memory distributed hashtables, effectively, with PowerShell administration. I showed this at TechEd in Dubai, it's pretty cool.
For composite apps, on the business tier, AppFabic gets you services to support Windows Workflow and Windows Communication Foundation (WCF) apps. That means, workflows and web services get supporting services for scale. (remember "Dublin?"). For all apps, you get nice instrumentation in MMC that will live alongside your IIS7 management snapins, so you don't have to run around in multiple places to manage apps.
Most of these links, training and sample, show Beta 2 today, but will be updated soon to the final bits, I hear. There's lot more coming, and I'll do my best to collect the info in as clear a way as possible.
If you're building BIG stuff of scale, as I did for 15+ years, AppFabric should prove pretty useful. I'm going to spend some time digging into it and I'll try to get the inside scoop from the team in the coming months. I'm also going to look into how well this all plays with Open Source libraries and subsystems.
Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. I am a failed stand-up comic, a cornrower, and a book author.
Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.