OOC: Okay, so basically, choirs are divided up into the vocal ranges people sing.
Soprano- girls who have higher singing voices
Alto- girls who have lower singing voices
Tenors- boys who have higher singing voices
Bass- boys who have lower singing voices
When it comes to the music itself, obviously they sing different parts of the song. Sopranos sing one thing, altos sing another thing, tenors sing another thing, and bass things another thing. Most of the time it's the same words on different notes, although sometimes they're different parts of the song entirely. Sometimes, we complicate the matter further (to get a better sound) by dividing it up more. This means (in our case), that there are three sopranos. Sopranos, Mezzo-Soprano 1, and Mezzo-Soprano 2. Not only does this enhance the comfort of the singers (so they're singing in ranges that are completely reasonable for them), but it makes the choir sound better because it gives more opportunities for harmony!
So choral arrangements can be SATB (meaning they have soprano, alto, tenor, and bass singers), SSAA (Soprano 1, Soprano 2, Alto 1, Alto 2), and more. Does that make sense?