Sun Ray Server Software 3 is what runs on the server and manages the communication which makes sure the Sun Rays can connect and get a login session (the session itself is just an ordinary session as provided by the underlying operating system). Sun provides this for Solaris and for Linux. We have been attempting to get it to run on Fedora Core 3.
A Sun Ray is a "thin client". That is to say, it is a box into which you plug a screen, a keyboard, a mouse and a network connection, but the desktop and applications which you see on the screen are actually running on a server in a machine room somewhere. (Given that, I'm not quite sure why the box is as big as it is, though it does have some extra magic in the form of a smart-card reader which means that you can disconnect your login session with its desktop and running applications and come back to it later, possibly on a different terminal.)