It reflects your mood. If you listen to sad music...you are. If you listen to happy music, you are...thus...who you are.
How does the type of music you listen to define who you are?
It can be hard to define if like me, you listen to a lot of non-related music genres.I listen to a lot of bands and my favourite music genres are rock music, electronica, gothic rock, indie music and "Britpop" music.I also like listening to 80s indie music and some rap-rock as well.
Continuing on Forest's answer, If you listen to crappy music, like Justin Beiber or Rap and Pop in general, you are a crappy person. If you listen to good music, you are a good person.