The master API has a somewhat confusing name for users that are used to traditional Unix terminology. Because the master is what steers a Salt infrastructure, the API that is used to configure it is called the wheel system. To put it clearly, Salt's wheel system bears no relation or resemblance to the wheel group found in many Unix and Linux distributions.
Like most of Salt, the wheel system is pluggable. However, while a few modules do exist, most of them will never be used by most end users. For our purposes, we'll focus on the ones that are used.
The part of the wheel system that is most commonly seen by administrators is key management. When keys are accepted, rejected, or deleted, that action is usually performed by the wheel system. The key module doesn't actually do much: it creates keys when necessary, moves them between directories as requested, and fires events along the event bus when doing so.