The name "nebula" is from greec (or latin) and means "dust and fog". The Milkyway contains a lot of dust and fog from hydrogen, helium and other gases. They appears like an instellar cloud on the sky, but they are not visible with the naked eye (except the Orion nebula). Nebulas are giant clouds in the space and some of them are several light-years in diameter. They are thrown out by explosion stars (like a supernova). Other are regions where new stars are beginning to form. Its mainly two types of nebulas. Reflections - and Emissions nebula.