What is a piece of music called when you have tune then something different again?

A piece of music called when you have a tune then something different again is called a rondo. A rondo is a musical composition that consists of a recurring theme (the "rondo theme") that alternates with contrasting sections (the "episodes"). The rondo theme is typically the main melody of the piece, and it is repeated several times throughout the composition. The episodes are usually more developmental in nature, and they can feature different melodies, harmonies, and rhythms. Rondos are often used in classical music, and they can be found in works by composers such as Mozart, Beethoven, and Schubert.