Head voice and Chest voice were discussed in this question about natural voice, but despite singing in choirs for nearly 9 years I have never heard of them.
Can someone provide a good description of them and what each one is used for?
Head voice, chest voice, throat voice, as well as their "register" version and falsetto are old and confusing names to describe an almost universal phenomena:
When singing from the lowest to the highest pitch than you can sing, the hollow parts of our body (chest, larynx, mouth, upper cranium) share and sustain differently the resonance of the sounds generated by the vocal chords.
One can feel these differences when singing but also when hearing them, because they give a different "colour" to the voice, because they amplify or diminish parts of the overtones of the voice.
Starting from the Middle Ages, it has been conventional to identify the successive most common ways of singing from low to high notes : chest , throat, head voices by analogy with a vertical scale of pitch, but physiological research shows that this is misleading.
These resonances can also hinder or ease the production of certain notes, and we can act upon them by muscular control of all the organs involved (control of the abdominal muscles, of the ribs, of the throat, especially the larynx, position of the jaw, etc.)
You can use these differences and this knowledge to emit more pleasant notes and extend your range. Most people naturally speak with a given type of emission, that is the most natural to them. With training they can speak comfortably in different modes and switch from one to the other.
You may surprise yourself for instance to be able to speak high notes one moment and not be able to do it again when you have sung a few lower notes in between, because you cannot recreate consciously the conditions for those high notes.
A large part of the classical "bel canto" technique consist in controlling this in a conventional way toward a few goals (essentially masking boundaries between resonance modes, choosing the best pitches to make the transition) and to reproduce classical voice models corresponding to typical opera role.
I feel it is easier to learn about this by having lessons with a teacher who has already mastered this and can concretely help you to discover it and use it.