Determining the Level of Sheet Music

[janyaporn]

I have a question that I cannot seem to find an answer to on google.  I am not a classical musician for the most part so I am not familiar with the "level" system.  In the past I had a teacher who referred to the music scores as easy, medium, hard, or difficult.  He was teaching me piano as a pop musician.

So my question is, how does one determine if a song is easy, medium, hard, or difficult?  Is it the key, chords, what? Please explain.

Thank you.