cPanel is designed to manage your website. You can setup or terminate databases, manage email accounts, view website stats, setup cronjobs, install scripts and do other tasks using cPanel.
WHM (Web Host Manager) is used to manage client accounts and is available for resellers only. You can setup or terminate client accounts, manage packages, upgrade or downgrade accounts, edit DNS records using WHM. All your clients will have their own cPanels to manage their websites. Use WHM to manage your client accounts only. If you need to manage your own reseller account, please use cPanel.