The sopranos of the choir are female and sing the highest parts. Their voice range is typically between a middle C and A above the staff. Most sopranos can sing higher, but this is the safe range for vocal writing. The range doesn't always determine the type of voice classification, however. Sopranos generally also have a brighter sound than altos. Sopranos are classified as a high voice for females.
Altos are female singers who can sing from a low F to a D one octave above middle C. Many altos can sing in the same range as a soprano, but their voice quality has a darker, thicker texture. The alto voice is classified as a medium voice for a female.
Tenors are in the high voice range for men. They sing between a B that's a little more than an octave below middle C and a G just above middle C. Their timbre is often thin like a soprano and has a piercing quality. Tenor lines are generally very stepwise and don't move much in the texture.
This is the lowest voice in the choir. It plays from an E below the tenor's lowest voice and a middle C. Basses generally have very round, deep voices that act as the foundation for a musical composition. Voice-leading conventions generally allow the bass to jump around much more than the soprano and other interior voices. It is also common to find the bass more than an octave from the higher voices.