In case you missed it, the evolution of hardware has caused an architectural change in Exchange 2013.. gone are the 5 Exchange server roles that we have all come to know and love since Exchange 2007. In Exchange 2013 things are a lot more simple with only two server roles, namely, the Client Access server role and the Mailbox server role. Don’t let the familiar names fool you though, there is nothing familiar about these roles. In Exchange Server 2013 there are two basic building blocks – the Client Access array and the Database Availability Group (DAG). Each provides a unit of high availability and fault tolerance that are decoupled from one another. Client Access servers make up the CAS array, while Mailbox servers comprise the DAG.
Ross Smith did a great job (obviously!) of introducing the new Exchange 2013 role architecture during the technical keynote at MEC 2012 last year.. if you have not already seen it, I highly recommend you watch it here. He also recently publish the following post of the Exchange team blog that covers the same topic.