According to my voice teacher, it is both the quality of your voice and where the breaks are in your range. However, as an earlier post said these breaks may shift with time and the better trained your voice becomes, the less noticeable those breaks become.

As for the quality of your voice, my teacher said a great thing that really stuck with me, so I will pass it on. I was struggling to have a more mature rich sounding voice and I brought in several recordings of female singers that I would like to sound more like.   He listened and said.... you've got to play the voice your given.... some people have a mazarati (quick light turns through pitch) and others have a cadillac (rich inertia). Better breath control and support have fattened the lower side of my range, but really what my voice does best is make clear bright sounds. I know longer think much about expanding my range, but rather strive to sing more fully in each part of my range.   Does that make sense?

Anyway.... I really recommend taking lessons. This spring I will be teaching a workshop which will synthesize some of what I have learned over the years and bring it to those that just want to sing a bit better in the folk scene.

All the best,

Claire