Default <server name> Accepts connections from Mailbox servers running the Transport service and from Edge servers.
Client Proxy <server name> Accepts connections from front-end servers. Typically, messages are sent to a front-end server over SMTP.
Default FrontEnd <server name> Accepts connections from SMTP senders over port 25. This is the common messaging entry point into your organization.
Outbound Proxy Frontend <server name> Accepts messages from a Send Connector on a back-end server, with front-end proxy enabled.
Client Frontend <server name> Accepts secure connections, with Transport Layer Security (TLS) applied.
FYI:
http://technet.microsoft.com/en-us/library/aa996395(v=exchg.150).aspx