Hello,
Fundamentally, both words mean exactly the same thing, though monitoring might denote more responsibility. The former word comes from the Latin 'monere' meaning to warn or advise, the latter from 'super-videre' meaning to watch over. Webster's Dictionary defines supervision as "the action, process, or occupation of supervising, a critical watching and directing (as of activities or a course of action)"; for monitoring, the definition is "to watch, keep track of, or check usually for a special purpose." Monitoring can be seen to be more specific in being aware of one particular matter, therefore.
Often in American schools, there exist milk monitors or hall monitors, who themselves may be placed under adult supervision, denoting how physically the adult is taller than the child. At some British schools, monitors act as ambassadors of the school, setting an example for younger cohorts, while again they are themselves under supervision.
In department stores, speaking to the supervisor will denote an employee who is stuck on a customer inquiry directing him or her to a more senior employee who has extra responsibilities while not directly monitoring the routine of the junior worker. In essence, though, both words mean the same and are to be treated accordingly.