In a scrum, who is a scrum master? What are his or her responsibilities?

 A scrum master is an expert in scrum processes who helps to keep the process organized and operating. The scrum master is not the boss; he or she is more like a coach or even a referee. The scrum master steps in when meetings go too long, when team members are misusing meeting time, when the product sponsor isn't doing his or her job, and when other situations are not working. The scrum master is also the guardian of team members' time. If distractions occur, the scrum master steps in to eliminate them. If someone other than the product sponsor attempts to change requirements or priorities, the scrum master negates that attempt.