With the release of Exchange 2007 and then 2010, Microsoft moved admins to a role-based model for deployment, with changes both to roles and services that admins should understand. Exchange 2013 takes that evolution even further.
The essential roles for any Exchange deployment include Client Access, Hub Transport, and Mailbox. As the roles orientation has deepened, Microsoft has also been consolidating them. In Exchange 2013, the old Hub Transport role, which routes incoming and outgoing messages through the hub-transport server, has been incorporated in the Client Access and Mailbox roles. The Transport service that ran on the Hub Transport server has been moved to the Mailbox role.
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Exchange 2013 brings with it other services to help move mail along. On the Client Access side is the new Front-end Transport service -- essentially a proxy solution for inbound messages from the Internet to a mailbox server, though it can be configured to relay messages from the mailbox server back out to the Internet. On the mailbox server, in addition to the Transport service (which handles mail queueing), Exchange 2013 provides the Mailbox Transport Delivery service (which accepts email from the Transport services over SMTP and converts it to RBC, then delivers it to the mailbox database itself) and the Mailbox Transport Submission service (which takes mail from the mailbox database using RPC and passes it off to the Transport service using SMTP).
The removal of the Hub Transport role isn't the only change in 2013 mail flow you should understand. Another change is the use of delivery groups to determine how mail will flow and route between servers in different Active Directory sites. In Exchange 2007 and 2010, mail routing was done based on Active Directory sites and the cost of Active Directory site links, though you could adjust the costs for Exchange links to override if you wanted.