a Rondo is a principal theme (sometimes called the "refrain") alternates with one or more contrasting themes, generally called "episodes", but also occasionally referred to as "digressions" or "couplets commonly used in the baroque/early classical period
A Sonata is a in summary, a composition of large scale works. (Usually divided into movements) that is a fundamental piece of the piano study
Variations is basically just a different form of the main melody in a piece. Very common in the baroque era. It takes the melody and puts it into a different key, or a different octave etc etc.
