A choir is a musical group of singers who perform together. People have been forming choirs for thousands of years, at least since the Ancient Greek period or even before this!
Choirs are often led by a conductor or choirmaster and singers are normally split into different parts called soprano, alto, tenor and bass, depending on how high or low they can sing. Some choirs might sing without an instrumental accompaniment (a capella). Others are accompanied by instruments like a piano or organ or by a whole orchestra.