A "thin client" is a networked computer in a client-server architecture, which depends on a central server for processing. It focuses on communicating input and output between the user and the remote server resulting in all significant processing occuring on the remote server.
A normal PC or 'fat client' does as much processing locally as possible and passes only data for communications and storage to the server.
Thin clients may be either purpose built devices (which are usually diskless and small) or older PC's which have had their life extended (because they are only carrying out minimal processing and not using their local disks). The applications which would normally be located on the pc are located on a 'virtual pc' which resides on the server and the only data passed 'down the line' relates to the minimal data needed for screen and keyboard/mouse communications (hence the 'thin' name).
The 'thin' features of this technology means that it is also ideal for remote (off campus) use. Also because the 'desktop' delivered is centrally controlled and maintained, it is less reliant on the pc that the remote user happens to be using which in turn, means that it is also less likely to be detrimentally affected by any problems or risks that the pc in question may have present.
The additional benefit is that your thin client desktop can be as easily accessed on the other side of the world as it can be on campus. All you need is access to an internet connected pc and, assuming a reasonable connection, you will be able to work.