There's quite a bit of information at
http://www.owlinvestigations.com/forensic_articles/aural_spectrographic/fulltext.html
perhaps even too much.
I haven't read all of the material in the above URL myself, but I'll give you my understanding of how a voice works and what identifiable features it has. I shouls warn you that I'm not an expert by any means.
A voice is a low-frequency "buzz" that is shaped by a time-varying filter. The frequency and composition of the "buzz" is not particularly unique, nor is it constant in normal speech. (This frequency controls the pitch of one's voice, which one can vary over a fairly wide range voluntarily).
What gives a voice its unique quality is the shaping imposed on the underlying "buzz" by the persons body, which acts as a time varying filter. The length and width of the person's trachea, the exact dimensions of their mouth, and other physiological features basically control this filtering action, and are all highly characteristic of the individual.