The GNOME System Monitor menu entry starts a program called gtop. gtop is a graphical version of a text program called top. It displays information about all the programs running on your computer. There are three ways to start it:
At the command line, type gtop.
main GNOME menu+Utilities->Systems Monitor
If you have the multi_load applet running on your GNOME panel, then you can start gtop by right-clicking on the applet and selecting the Run gtop option.
UNIX and UNIX-like systems (Linux, Solaris, *BSD and so on) are multi-tasking, which means that they can run more than one program at a time. Each program can run one or more "tasks", or processes. All these processes share processor (CPU) time and memory. Whilst only one process can talk to a CPU at a time, the processes take turns at this which are very short. The result is that all the programs on the machine can proceed. gtop produces one or more constantly-updated windows which show you snapshots of the state of your system. The display can show you:
processes: which processes are currently the most active and what they are doing;
memory: how much memory is available and how much is in use;
filesystems: how much of the filesystem is full or empty.
The gtop window also shows some additional system information at the bottom. In all windows, a menubar and toolbar are visible. These let you customise the current display of information or switch to a different display. In addition, you can get more information on a particular process by clicking on the name of that process in the process window.
By default, gtop will begin with the first window: processes.