Anyone who has ever played any multiplayer game seriously to any degree will have those days -- days when it seems like you're simply getting destroyed one game after another. On days like those, Blizzard's new custom game -- Starcraft Master -- are a godsend, providing you with the chance to increase your skillset without the intensity of a multiplayer match.
Starcraft Master is a series of challenge modes designed to "tax your knowledge and your ability to control your forces." Think of them as super advanced tutorials that exist to refine your existing skill set and push things up a notch.
These challenges will apparently "demand your utmost concentration to prevail" and "help you hone your unit micro-management skills in situations which mimic real battles". The challenges range from "“kiting” a single unit, to managing incoming damage and maximizing unit abilities, to controlling a mix of units to obtain victory over a superior enemy force".
You can find more information on this new challenge mode here.