Exchange 2013 has two Roles:

1. Mailbox Server Role
2. Client Access Server Role

Each organization requires minimum one Mailbox Server Role and Client Access Server Role.

Mailbox Server Role Includes:

1. Client Access Protocols
2. Transport Service
3. Mailbox Databases
4. Unified Messaging (Except SIP Redirection)
5. Handles all activities for active Mailboxes on that Server

Client Access Server Role Includes:

1. Authentication
2. Redirection (limited)
3. Proxy Services for HTTP, POP, IMAP and SMTP
4. Thin and Stateless Server
5. Does not do any data rendering
6. Nothing is queued or stored here (except diagnostic logging)