Exchange server 2013 consists of two server roles, the Mailbox Server (sometimes referred to as the back-end) and the Client Access Server (sometimes referred to as the front-end). All clients connect to the CAS Server and the CAS Server proxies the request to the appropriate mailbox server.
It is possible to install the server roles on dedicated servers, multiple Exchange 2013 CAS servers with a hardware load balancer and multiple Exchange 2013 Mailbox servers with a Database Availability Group. This is the preferred way for large companies with lots of mailboxes, lots of servers and maybe multiple (global) datacenter. To be honest, this is where Exchange 2013 is designed for. But it is also possible for smaller organizations to install just two Exchange 2013 server with both roles on it, a DAG for mailbox resiliency and a hardware of software load balancers for the protocol resiliency.