The main tune of a piece of music. Melody focuses on the succession of pitches and pitch sequences to create something pleasant to the ears.
Harmony:
Combination of simultaneous pitches played together. Harmony focuses on the vertical aspect of music, or the different notes heard at the same time.
Rhythm:
the pattern of strong and weak beats in a piece of music. Rhythm focuses on the temporal aspects of music such as the duration of notes and rests, and the placement of accents and syncopations.