cPanel allows you to manage websites, e-mail addresses, databases, technical features (php & other programming languages) and a few clicks. You can give access to your customers to make them stand alone.
WHM (WebHost Manager) is the server administration panel that allows the configuration of services like MySQL, Apache, PHP, FTP, mail. This control panel allows you to create your own web hosting packages for your customers.
WHMCS is a turnkey solution in customer relationship management, billing and support. It also automates the account creation process using the cPanel API. The WHMCS solution is included from the M pack.