Testing of techniques through practice with a partner or against an opponent.
In judo, sparring is restricted to grappling techniques and called randori. It is considered an essential part of the progression of waza from uchikomi to shiai.
In karate, sparring is usually restricted to some set of striking (and sometimes limited grappling) techniques and is called kumite. It is considered one of the three pillars of training, along with kihon (basic techniques) and kata (forms).
Questions involve sparring tactics and strategy, etiquette and rules, application of technique, and any of the problems that naturally arise when friends punch each other in the face.