When you install Reporting Services, we create a few directories:
Most of these are fairly self explanatory. LogFiles... well we put our log files in there. ReportManager contains the Report Manager application (what you get when you browse to http://<server>/reports) and ReportServer contains the Report Server application which is the proper 'Report Server' which implements our Web Service and HTTP handler endpoints... it is what you get when you go to http://<server>/reportserver.
So what about that RSTempFiles thing? Well it contains files that are temporary of course!
These temporary files can be broken down into a few categories: