WHM is your reseller hosting control panel where you will create and manage hosting accounts (cPanel accounts) for you and your clients. A hosting account is needed to host domains and websites. Each hosting account has its own control panel (cPanel), username, and password. Note that the username and password are the same for your reseller account's WHM and cPanel. You can log into a hosting account from within your WHM and also by visiting the cPanel URL and using the login username/password of the respective hosting account.
cPanel is where you will create/edit email accounts, change their passwords, set mailbox quota (storage size), add/manage domains, manage website files, create/download/restore backups, etc. It is the hosting control panel for your and your clients' domains, websites, and emails.