A nebula, Latin for "cloud," is a difuse mass of various materials located within a galaxy. Nebulae are extremely rare in the space between galaxies or intergalactic space. They are classified based on their reaction to electro-magnetic radiation or EMR, i.e., radio waves, microwaves, visible light, ultraviolet light, X-rays, etc.
A nebula's response to EMR is dependent on its composition and temperature. If it consists primarily of dust particles such as sand and carbon, it will absorb all EMR except radio waves and microwaves. These clouds are called dark nebulae. Frozen gas crystals are usually present in them as well.
Nebulae consisting of gaseous molecules such as hydrogen, water vapor, carbon dioxide, methane, etc., absorb specific wavelengths of EMR and are transparent to other wavelengths. They are referred to as molecular clouds. Every molecule has a unique "spectral" signature, i.e., the wavelengths of EMR which it absorbs are specific; others pass through. This allows scientists to identify nebular constituents at great distances. Molecular clouds are not found near stars but in colder areas of interstellar space. They are mostly transparent to visible light because they do not contain liquid or solid material.
At low temperatures, water, carbon dioxide, methane, ammonia, etc. form tiny crystals. These reflect some EMR wavelengths, especially visible light, and produce reflection nebulae. The blue haze around some stars is an example of this type of nebula.
Near sources of intense radiation, such as hot young stars, ionized gasses exist. These gasses are too hot to form molecules and exist in a plasma state which is a mixture of energetic free electrons and positively charged nuclei. High energy external EMR (ultraviolet and xray) is absorbed by the plasma's electrons and re-emitted at longer wavelengths as the electrons recombine with ionized nuclei. Basically, high energy EMR received is transformed into lower energy EMR and re-emitted. Some of this energy is visible. Red hydrogen alpha and green oxygen III emissions are examples. These "transformers" are called emission nebulae.
The term nebula is also used to describe clouds of material involved in various cosmic proceses. Planetary nebulae and supernova remnants are examples of this usage.
Editorial Comment: Study Latin, you will not regret it.