Here's a songwriter's perspective.
I guess you could say that a song is made up of lyrics, chords, melody, vocals, harmony. Those are the basics that make up a song, but what is really in a song.Someone's entire life can be in a song. As a songwriter, it seems like I am letting everybody read my diary. Even though my songs are not necessarily about my life, they come from feelings and emotions I have had at one time or another.
Some songs are written in five minutes and some take years to complete. I think it depends on my intentions for the song and how it comes to me. A song takes on a life of its own from conception of the idea to maturation to the finished product...if there is such a thing. I have read that great songs aren't written, they are re-written. I think of them as a gift from somewhere out there in the Universe.
So, what's really in a song? Emotion, passion, love and feelings, because when you hear a great song, you feel those things.