Function and use of system tray

My question doesn't involve a problem I'm having, just looking for information. I would like to know the function of the System Tray in the task bar and how I can control what appears there. I'm using Windows 98 2nd Ed. on a XPS T-500. I can't seem to find any documentation on this in my Windows manual and Microsoft refered me to Dell since its an OEM machine.

Thanks for your time.