When you play or sing three or more pitches together you are performing a chord.
If you are performing a unison melody, this is an example of monophony.
When music has many overlapping melodies, this is an example of polyphony. (Singing in a round is an example of polyphony.)
If you are singing a melody with an accompaniment, your song has harmony.
The 3 common elements necessary for a mash-up or partner song are harmony, beat, and style.
When you perform two different songs at the same time, you are performing a partner song.
When you weave two songs together to create a new song, you are singing a mash-up.
If a song has a verse and refrain, we say the form is AB. "Downtown" is an example of an AB song.
If a song is ABACA, we say the song is in rondo form. The Mouret "Rondo" is an example of a rondo.