Client access role
Very similar to FE role in Ex2000/2003. Handles OWA, POP3/IMAP and OMA. Does not handle inbound/outbound smtp nor IMF.
Edge transport role
Essentially a FE SMTP relay server to be placed in a DMZ and also does anti-spam. Has to either not be a member of the domain or member of a seperate domain then the other Exchange servers. If you use Postfix or some other relay server than this role does not need to be implemented.
Hub Transport role
This role does the traditional categorizer lookups for message flow that used to be part of the BE role in Ex2000/2003. The server side part of MAPI has been changed and is now called the "(information) store driver". New feature message compliance allows native archiving policies and among other features system-wide footers/disclaimers to be added. The Edge transport agent can be installed to allow it to also act as a FE SMTP relay server also.
Mailbox server role
As BE server role in Ex2003 but can now be a true DB-only server.
Unified Messaging role
Uses a PABX to provide additional telephone features such as accessing emails and cancelling meeting requests etc. over the phone and emailing VMs to users. This feature is about 5 - 10 years late as most users now have emails on thier phones and a lot of companies deploy PBXs that already send VMs via email.