What is the difference between WHM and WHMCS?
WHM and WHMCS are two, completely separate products made by different companies. WHM is part of the cPanel & WHM product made by cPanel, LLC, while WHMCS is made by WHMCS Limited.
WHM provides a set of features to manage your cPanel server, its services, and its accounts. WHMCS provides automated functionality for account management, including billing services.
You can learn about WHM features here: cPanel & WHM® Features.
You can learn about WHMCS features here: Web Hosting Automation.