The adult human voice can, normally, be placed into one of the following vocal categories, listed from highest to lowest:
Contralto or Alto,
The first three categories are usually women's voices, and the last three, men's. There are generally two exceptions to this, being a male voice called a counter-tenor, which could be considered the Alto of the vocal family, and in some women's choirs, there are female Tenors.
In general, vocal sound production can be put in 2 categories, Head Voice, and Chest Voice. The Head Voice is normally attributed to a female singer, whose vocal cords are stretched "thin". This is by no means a solid definition, as the speed of air and spacing between vocal chords contribute in affecting the dynamics and pitched of vocal sound production. In a trained female voice, this head voice or head register is used for most of the vocal range.
When the vocal cords are shaped to become thicker, or 'fatter', the tone produced is considered to be a Chest Voice, or Chest Register. Both Male and female voices are capable of producing a chest voice. In trained voices, this Chest Voice can be considered a distinction between male and female, as a male singer will produce nearly all his range using the Chest Voice, where a female singer will only use the Chest Voice for the bottom 5th or so of her range. It becomes obvious then to assume that a soprano will use more head voice than mezzo-sopranos, or altos, as a natural extension of range capability.
There is a third vocal production technique called Falsetto, that isn't used often in orchestral choirs, but worth a quick mention. As the difference between this almost childlike tonal technique and normal vocal production is so great, they are rarely mixed. The Falsetto is more correctly defined as a transparent tone range above that which is considered the male tenor register. When performed by a skilled male singer, the difference in tone between a natural Tenor register and Falsetto register can be so close as to produce no discernible difference, and this can be useful for the composer as an interesting extension of the male vocal range. However the composer should use some caution, as not all Tenors can produce this effect successfully.