Web Host Manager, commonly know as WHM is a server management interface that offers administrative control over cPanel accounts.
WHM is a software developed by a company called cPanel, Inc.
WHM has two administrative levels by default. The first is the Full Server Administrator who is the root user and has complete control over the server. The second is the Reseller user who doesn’t have complete control over the server but can manage cPanel accounts. This level is for someone who want to manage a pool of clients from a single administrative portal or an entrepreneur who desires to start a web hosting company but their budget is limited or may be they want less work dealing with the server, since everything is taken care of by those who have root privileges.
By use of plugins, smart system admins are able to come up with other administrative levels namely; Master Reseller and Alpha Reseller. A master reseller can resell reseller accounts and an alpha reseller can resell master reseller accounts.
Below is an image of how WHM looks like