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.
WHMCS does automate some tasks in WHM, however, they are independent products.
You can learn about WHM features here: cPanel & WHM® Features.
You can learn about WHMCS features here: Web Hosting Automation.