Question: Windows Azure AppFabric Access Control Service supports Federated Security. Errr… what exactly is Federated Security?
Answer: To understand the purposed of the Access Control Service you need to first understand Federated Security. My colleague Steve has an excellent Federation Primer which I highly recommend you read.
Then check out this 10minute video from Steve on Using Active Directory and AppFabric ACS
Note that the Access Control Service is just one part of AppFabric