The Web Host Manager (WHM) is the account management panel that allows the hosting provider to manager their customers' accounts.
Creating new accounts and adjusting the limits for an account are just two examples of tasks that would be performed in WHM.
cPanel is the site management panel where each individual account holder controls the settings for their particular account.
Creating e-mail accounts and managing their files for a particular domain, for example, would be tasks performed in cPanel rather than WHM.