In Exchange 2007, when you installed Exchange, you had to specify what server roles you wanted to put on the server. This allowed you to move specific roles to seperate hardware and move the load over several servers. Those roles included: Mailbox, Hub Transport, Client Access and Unified Messaging.
Changes in Exchange 2013. Now, Exchange only has two roles. It has the Mailbox and the Client Access roles only. They have taken all the roles besides the client access role and have moved them back to the Mailbox Role.
The Mailbox server includes all the traditional server components found in Exchange 2010: the Client Access protocols, Hub Transport service, Mailbox databases, and Unified Messaging. The Mailbox server handles all activity for a given mailbox. The Client Access server provides authentication, redirection, and proxy services. The Client Access server itself doesn’t do any data rendering. The Client Access server is a thin and stateless server. There is never anything queued or stored on the Client Access server. The Client Access server offers all the usual client access protocols: HTTP, POP and IMAP, and SMTP. [SOURCE]
This creates a couple of gottchas that need to be addressed. First, they no longer support direct RPC connections to Exchange, you have to use RPC over HTTPS. Second, Outlook 2003 is no longer supported with Exchange 2013, you must have all clients running newer version of Outlook.