It really depends on the server's function: You can literally start with no roles installed. The one function you mention, "Remote Desktop Host", is actually called the "Remote Desktop Services" role, and the Remote Desktop application in Windows is used to use this service. This role was formerly known as "Terminal Services".
Even without that role, Server 2008 R2 comes with the ability for two people to simultaneously log in using Remote Desktop, however, if you wish to use that role for additional simultaneous users, you will need to install the role, and purchase separate Remote Desktop Services Client Access Licenses for each user, in addition to the regular Server 2008 user CAL's.
When you purchase these Remote Desktop Services CAL's, you will be able to enter them into you Remote Desktop Services license server. Without this, you only have a 120-day trial period, which is fully functional, but will cease to work after that without the license server and licenses entered into it.
As the others said, if it requires any roles, which I doubt based on a Google search, it will probably install them automatically, or the documentation will spell it out. Most ordinary applications do not require special roles unless they are integrated in Active Directory, then they require those related roles. I believe the answer to your basic question is "none".