When we hear music there are several components in it that we often do not differentiate. However, from the base instrumental sounds to tunes as well as background instruments and voice, these are some of the several aspects of a song that we hear. If we sit to think what a tune is, it could be a song or a main underlying musical theme. However, in most cases melody would refer to an underlying theme of a musical piece.
If we say that someone played a little melody, it would be akin to using a single finger on the keyboard. However, a tune that sounds delightful would be one that is played for by both hands. If the word song is used instead of the word tune, it would mean that a tune was accompanied by lyrics.
When we talk about melody, it does not need to be in harmony. There can be different melodies included on a song being sung by a choir. For instance, the sopranos would sing the melody in the fifth measure while if you ask the choir what tune is being sung, you would get puzzled looks or someone might play the A chord on the piano.
Those who dabble in music appreciate the fine differences that exist in the different components that make up music. Then again, the quality of notes created with different musical instruments also brings in different qualities and depth and that is what enriches music compositions in different ways.