ACS is short for Windows Azure Access Control Service
Active Directory Federation Services (ADFS) is a standards-based web Single Sign-On federated identity which implements claims based authentication across forests.
"It seems possible to have adfs as the ip-sts send an assertion to more than 1 acs url based upon the acs url or acs index in the authnrequest as long as they are listed in the relaying party trust endpoints list"
from question "Using ADFS, SAMLResponse is not sent to ACS URL appearing in SAMLRequest [authnRequest]"
"Adfs has more powerful claims transformation capabilities than acs"
from question "AD FS 2.0 with PingIdentity / AppFabric Labs ACS"