The Web Host Manager (WHM) is the account management panel that allows the hosting provider to manage their customers' accounts (for Resellers). 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 (you, the website owner) controls the settings for their particular account. Creating email accounts and managing their files for a particular domain, for example, would be tasks performed in cPanel rather than WHM. See our cPanel section in the Knowledgebase.