GNU/Linux is bursting with information about the system on which it runs. The system's hardware and memory, its Internet link and current processes, the latest activity of each user -- all this information and more is available. And, despite such desktop tools as the KDE Control Center or GNOME's System Monitor, the easiest place to get all the system information available is still the command line.
In many cases, you can view system information via specific commands. Some of these commands are written specifically to give information, while other commands that are mainly intended to alter the system in some way just happen to include parameters for viewing the current state of the system.