"All are of the dust, and to dust all return."
Nebulae are unique. Like people.
A nebula (Latin for "fog" or "cloud") is a gigantic cloud of dust and gas in space. Actually, the word "gigantic" does not even begin to describe the size achieved by many nebulae, with the most gargantuan of nebulae being thousands or even millions or light-years in their maximum dimension (1). They are truly vast. However, although denser than the space around them, nebulae are far less dense than any vacuum that can be made on Earth - thus, a nebula cloud the size of Earth would only weigh several kilograms.
Nebulae are both the birthplace and graveyard of stars. When the interstellar dust and gas of a nebula clumps together to form denser regions, a process that attracts more dust and gas, that region can eventually become dense enough to form a star. However, when a star goes supernova, which is an apocalyptic stellar explosion, the dust and gas thrown off from the supernova can also form a nebula.
Despite their uniqueness, nebulae can be classified into emission, reflection and dark nebulae (this is not the only way to classify nebulae, but it is the way that seems the most logical to me).
Emission nebulae emit their own light. They are composed of dust and ionized gases; the temperature is high enough that the gases have become ionized, meaning some of their electrons have been removed. The most common source of the ionized gases is one or more stars sitting within the nebula. Emission nebulae are diverse, and as such they can be sub-classified into HII region, planetary, and supernova remnant nebulae.
HII region nebulae (pronounced "H two") are massive clouds of dust and gas in which star formation is taking place. A single HII region can give rise to thousands of stars over several million years. These nebulae may be of any shape, often distorting themselves into complex and intricate shapes. The award for most reputable HII region nebula would probably go to the Orion nebula, discovered in 1610 by the French astronomer Peirescius. The Orion nebula is one of the brightest nebulae in the sky, visible to the naked eye, although it is only 24 light years across, which is rather ordinary compared to many other nebulae. It forms the middle "star" (which is of course not really a star, it is the Orion nebula) in Orion's sword, in the constellation Orion.
Orion nebula, one of the brightest in the sky (2).
Helix nebula, likened to the "Eye of Sauron" (3).
Crab nebula, a remnant of the SN1054 supernova (4).
Rather than emit their own light, reflection nebulae mirror the light of nearby stars. Like emission nebulae, they are composed of dust and gases, but since they do not emit light, only reflect it, the temperature does not get high enough to ionize the gas. Reflection nebulae are usually blue, as blue light is more efficiently scattered, or reflected, than red light (the greater efficiency of blue light scattering is also the reason the sky is blue). The Witch Head nebula (more formally known as IC 2118) is an enormous reflection nebulae illuminated by the nearby supergiant star, Rigel. This nebula approximates the sideways portrait of a wicked witch, pointy nose and all. The Witch Head nebula is 70 light years across and lies within the constellation Eridanus (although its illuminator, Rigel, lies within the constellation Orion).
Witch Head nebula, reflecting Rigel's light (5).
Finally we come to the dark nebulae, clouds of dust and gases that are so dense that they obscure the visible wavelengths of light emitted from any structures behind them, including stars as well as emission and reflection nebulae. The reason dark nebulae are so "dark" is that they contain dust particles coated with frozen carbon monoxide and nitrogen, which block all visible wavelengths of light. Dark nebulae often form convoluted, irregular shapes, the largest of which are visible to the naked eye and appear as a dark patch in the sky against the brighter background of packed clusters of stars. Dark nebulae themselves can form stars. A well-known dark nebula is the Horsehead nebula, which resembles a horse's head and is 13 light years across. It lies within the constellation Orion.
Horsehead nebula, blocking all visible light behind it (6).