A master class is a class given to students of a particular discipline by an expert of that discipline—usually music, but also painting, drama, any of the arts, or on any other occasion where skills are being developed.
The difference between a normal class and a master class is typically the setup. In a master class, all the students (and often spectators) watch and listen as the master takes one student at a time. The student (typically intermediate or advanced, depending on the status of the master) usually performs a single piece which they have prepared, and the master will give them advice on how to play it, often including anecdotes about the composer, demonstrations of how to play certain passages, and admonitions of common technical errors. The student is then usually expected to play the piece again, in light of the master's comments, and the student may be asked to play a passage repeatedly to attain perfection. Master classes for musical instruments tend to focus on the finer details of attack, tone, phrasing, and overall shape, and the student is expected to have complete control of more basic elements such as rhythm and pitch. The value of the master class setup is that all students can benefit from the master's comments on each piece.
Many concert performers have given master classes, starting with its inventor Franz Liszt and including such greats as Yehudi Menuhin, Isaac Stern, Itzhak Perlman, and Vladimir Horowitz. Often, a touring performer will give a master class the day before, or the day of, their performance in a particular city. Giving a master class before a concert provides both artistic stimulation for the performer and a means of obtaining a larger audience.
Some musical theatre composers will also give master classes to college students studying performance.
Some speciality classes may be referred to as 'mini master classes'. These can involve short, faster lessons on a new subject. Students, typically experienced in one discipline, may attend these classes to learn the basics of a new, related discipline.