Short answer: cPanel is a commercial web-hosting control panel (with WHM for the server administrator) that manages websites, email, databases, DNS and SSL through a browser. On a VPS it is an optional, separately licensed add-on that turns the server into a classic hosting environment.
cPanel explained
WHM (WebHost Manager) controls the server and creates accounts; cPanel is the per-account interface for files, email, databases and one-click apps. Licences are priced by the number of accounts, so check whether a VPS offer includes the licence or charges for it monthly.
If you host one or two sites and are comfortable with the command line, a free panel such as CloudPanel or HestiaCP, or no panel at all, is usually enough.
See it in the data
Frequently asked questions
Is cPanel free on a VPS?
No. A few providers bundle the licence into managed plans; otherwise it is a monthly add-on.