At one client a would be Scrum Master told me "I coach the team like a football coach. I tell them what to do." No, the ScrumMaster is not the coach in the team sport sense of the word. If we want to equate it the sports world the almost perfect parallel is the referee. A referee makes sure the teams play by the rules and that nothing interferes with the game. Scrum Masters do coach the team in a mentoring sense, but they don't call the plays.
Let's do a quick side by side comparison:
(specifically a football referee)
|Enforces the official league rules of play for the game.||Enforces the agreed upon rules for the team's adopted Scrum process.|
|Does NOT solely dictate the rules, but contributes to their definition along with team owners, coaches, player representatives, etc.||Does NOT solely dictate the rules, but contributes to their definition along with management, Product Owners, Scrum Team members, etc.|
|Ensures the progress of the game is correctly communicated via mechanisms like the down marker, the score board, etc.||Ensures the progress of a Sprint is correctly communicated via mechanisms like a sprint task board, burn down charts, etc.|
|Ensures the timely execution of each play by keeping track of the play clock.||Ensures the timely execution of each Sprint by keeping track of the time boxed activities.|
|Does NOT tell the teams what play to execute.||Does NOT tell the Scrum Team was tasks to execute.|
|Does NOT determine the game plan for the team to implement.||Does NOT determine the features a Scrum Team will implement.|
|Resolves any conflicts and removes any field obstructions to allow the teams to play the best game they can possibly play.||Resolves conflicts and removes obstacles so the Scrum Team can develop|
|Ensures that the plays and game plan executed by the team follow the league rules.||Ensures that the tasks (Sprint Backlog) and features (Product Backlog) do conform to the agreed upon rules.|
|Is not held accountable when a play is botched if all the rules were followed. (Not counting bad calls!)||Is not held accountable for a team missing it's goal when all the rules are followed. (Although a good one helps find out why it was missed.)|
|Wears an ugly black and white uniform.||Is free to wear whatever they wish. (Within reason that is…)|
Alright, so it is not a 100% perfect match, but the analogy is pretty darn close. The main point is that the ScrumMaster does not lead or control the team. The Scrum Team does that itself. The ScrumMaster does not manage the features and determine which ones the team should work on. The Product Owner does that.
So if you do intend to be a ScrumMaster, pull on your white and black striped shirt and get your whistle ready!
BTW - If you want some canned definitions for the role check out what Mike Cohn and Ken Schwaber have to say. Also, if you ever wondered what the official duties for a football referee were, you can see the NFL's explanation here.