They can be many things, scar tissue from infections, they can be cancerous nodules or non-cancerous nodules. I believe they classify anything under 3 cm as a nodule. Anything over 3 centimeters is a mass. I could be wrong about the dimensions but that's what I remember reading.. almost everyone has some sort of nodules in their lungs, generally they're harmless. Usually from infections or living in cold climates. There's probably some people on here that know far more about them, but I hope that helps a little!