how to monitor the system status?

FROM : Scott.D.R
DATE : Wed Mar 05 23:13:23 2008

Hi everyone.
As the titled mentioned, How to monitor the system status?
Mac OS X provided a very powerful application "Activity Monitor", 
which can let you inspect the system status such as CPU usage, disk 
usage, network usage and so on. But how to retrieve these information 
using APIs?

Thank you very much for any guidance.

Best regards.