Basically when two classical pianists get together in a room/concert hall and take turns playing the piano. One may play for example a Chopin Etude and then the other competitor may play a Liszt etude in rebuttal. OR one may play a Mozart Sonata and the other may play a Beethoven Sonata in response.The winner of this "duel" is determined by the unanimous vote of the audience, which would loosely indicate which pianist has the better technical ability, and musicianship.
Hey, did you just see that Piano Duel? That young pianist was just flamed by his competitor's exquisite interpretation of Beethoven's Waldstein Sonata.