iStat can view the stats of a computer running Mac OS X, Linux or Solaris and the appropriate iStat Server application. For more information about iStat Server, please refer to the specific documentation for each platform.

The CPU section shows information on the current state of your CPU(s) a graph for the last 171 seconds of activity. For more information on the meaning of user, nice, system and idle, please read our iStat terminology document.
The Memory section shows detailed information about memory usage on your remote computer. For more information on the meaning of active, inactive, wired and used, please read our iStat terminology document.
The Disks section shows detailed information about all the hard drives connected to your computer. CDs, DVDs and removeable media aren’t shown (that’s intentional!).
The Network section shows information about all your network interfaces and includes realtime stats for upload, download, highest peak for either upload or download, and a graph for the last 171 seconds of activity.
The Temperatures section shows the temperature sensors in your remote computer. Please note that some PPC Macs are not supported by iStat. Please also note that we do not have information on the normal operating temperatures—this is best obtained from Apple or Intel.
The Fans section shows the fan sensors in your remote computer. Please note that some PPC Macs are not supported by iStat. Please also note that we do not have information on the normal operating speeds—this is best obtained from Apple or Intel.
This is the length of time your remote computer has been on since the last restart. Please read this wikipedia entry for more information on uptime.
The values found under the “load” section measure the average system load during the last one, five, and fifteen minute periods. Please read this wikipedia entry for more information on load averages.