Originally, the word “nebula” referred to almost any extended astronomical object (other than planets and comets). The etymological root of “nebula” means “cloud”. As is usual in astronomy, the old terminology survives in modern usage in sometimes confusing ways. We sometimes use the word “nebula” to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas clouds. More strictly speaking, the word “nebula” should be reserved for gas and dust clouds and not for groups of stars.
By order in which they appear from top to bottom, left to right, here are the main types and some provided examples for visual reference:
Planetary Nebulae: Sh2-188
Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably evolve a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.
Dark Nebulae: LDN 1622
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.
Emission Nebulae: NGC 896
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation.
Reflection Nebulae: NGC 1333
Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.