Activity Monitor is a very useful free utility provided by Apple as part of OS X.
We found a masterclass on how to use it at a (non-CATUG) blog by Roland Banks:
It is handy to leave it running and change the dock icon to monitor your CPU history. Until you need more info, the app window can either be hidden (using the Hide command from the Dock), or simply left out of sight behind other apps.
This page was added by Neville Reid in December 2016.