WHM and cPanel are two different panels, we will proceed to explain each one of them:

WHM (Web Host Manager) is a control panel, where we can register and manage different hosting accounts that we generate for our sites. WHM has control over all the hosting accounts that we create, being able to modify their characteristics, users, ...

cPanel is the control panel of a specific account. In this panel, we can create email accounts, FTP accounts, manage databases, ...

So to speak, WHM is the server control panel and contains several hosting accounts, and each of these accounts has a cPanel panel for them.