Copyright Violations on Compilations?

I truly LOVE the compilations and have been considering making one myself.

Out of curiosity, is any music fair game? I hear popular classical/soundtrack music in the background of compilations and am wondering about copyright issues.

I don't want to break the law or get hammered with some kind of lawsuit. Any info is appreciated.

Thanks.

My understanding is that it's OK to use the music as long as the compilations are not being bought or sold.

In Christ,

Ron