In this video, we take a look at Isolated EPGs. By default, endpoints within the same endpoint group are free to communicate with each. However, there are use-cases where you might want to group a common set of devices, allow them to communicate to an restricted set of services, but not allow them to communicate with each other (i.e., backup interfaces on servers, web-servers in a DMZ).
Prior to ACI, the common way of doing this was with PVLANs. Isolated EPGs is very similar to the implementation of PVLANs, but not quite the same.