What is the difference between a chamber choir and a church choir? Do both sing hymns at a church service?
The currently accepted answer is essentially correct, but it doesn't consider the context you've mentioned in a comment:
A local church have a chamber choir and I was wondering if such a choir would sing at a church service.
The answer is probably yes. While it is true that "chamber choir" is typically used for small choral ensembles that present concerts rather than singing in church services, the more important element of the definition is the size of the ensemble. In a church with a choral program that comprises more than one choir, it would not be exceptional in the least to designate the smaller of two (otherwise similar) groups as the "chamber choir". Such a group would likely participate in services.
Chamber choirs play concert performances, church services aren't typically considered "concerts."
A chamber choir is a small or medium-sized choir of roughly 8 to 40 singers (occasionally called 'chamber singers'), typically singing classical or religious music in a concert setting. (This is distinct from e.g. a church choir, which sings in religious services, or choirs specializing in popular music such as a barbershop chorus).
My deleted answer was: "Basically, one fits in a chamber and the other one in a church ;-)" To clarify: That's exactly the difference, as both can perform any kind of music, because the number of voices in any choir part rarely exceeds the number of people in a chamber choir and almost never exceeds the number of people in a church choir. Rare examples are contemporary music parts like those from Ligety or Stockhausen. If administrator "Tim, Dom" (sic!) lacks the knowledge of music that is asked for here, he better go and troll contributors elsewhere. Thank you.