Your reseller account comes with two control panels, cPanel and WHM. WHM is your reseller control panel and this is where you manage all of your sub-cPanel accounts for your clients. From here you can create additional accounts, manage resources and limitations, add additional features and so on. This is essentially a top-level view of everything under your reseller account.
Each client or cPanel account that you create in WHM, has its own separate cPanel login. cPanel is where you manage each individual site or account, and this is where you create mailboxes, install WordPress, or check your site statistics.