WHM is Web Host Manager. It allows resellers to control and manage shared hosting clients quickly and efficiently.
For more information about WHM, please see WHM documentation.
cPanel is a world-class control panel designed for website hosts (i.e the customers of resellers). It allows the clients of web hosts to manage and control their shared hosting accounts with a simple yet intuitive web UI (user interface).
For more information about cPanel, please visit their website.