A group of teens and/or preteens who get together, usually in a church to discuss God and matters of life. they are usually taught by priests / ministers who play fun little games with them while at the same time really indoctrinating (brainwashing) them to the values fo their religion, i.e. chastity, abstinence, chastity some more, purity. They usually have a general focus on "saving oneself" - trying to convince the teens not to have sex.
Hey, are you going to youth group tonight?
No, I think they are talking about chastity again.