Because early telescopes were not powerful enough to resolve many globular star clusters, and the status of galaxies as island universes had not yet been recognized, globulars and galaxies were characterized as nebulous, or cloudy. Classes of nebulae are emission nebulae, reflection nebulae, planetary nebulae, supernova remnants, and dark nebulae.
Emission nebulae shine by internally produced light.
They are often the birthplaces of stars. M42 in the sword of Orion is the best-known emission nebula and is visible to the naked eye as a cloudy patch. Binoculars and telescopes give very nice views of this wonderful object. Reflection nebulae shine by reflecting light from nearby stars. Most are very faint, but M78 in Orion is visible in any telescope.