The word nebula comes from the Latin word meaning cloud. If you’ve ever looked up and observed a nebula, you’ll understand why: so many nebulae look like clouds against the dark, starry sky.
Originally the word nebula was used to describe anything that existed beyond our own galaxy. However, nebulae are interstellar clouds of dust, hydrogen, helium and ionized gases. Many of these nebulae are star-forming regions.
There are several types of nebulae including planetary nebula, emission nebula and dark nebula and reflection nebula.
Planetary_nebulae have nothing to do with planets. This type of nebula forms when a star comes close to the end of its lifespan. Fusion of elements like helium force the inner part of the star to heat up and a stellar wind lasting thousands of years is created, dispersing the outer layers of the star. The remaining core is still heating up the distant gases, causing them to glow.
Emission Nebulae are clouds of ionized gas which emit light. Planetary nebulae are a kind of emission nebula, but there are also other emission nebulae including HII regions where new stars are born. The Orion Nebula and Eagle Nebula, for example. HII regions are regions that have a central cluster of new stars, which heats the hydrogen in the nebula, making the hydrogen emit light.
Dark Nebulae contain incredibly small grains of interstellar dust comprised mostly of graphite, ice and silicates. Most dark nebulae are part of giant molecular clouds and come in many different shapes. They do not radiate light: they obscure visible light from us when they are between us and other stars.
Reflection Nebulae are clouds of dust and gas which reflect the light from a nearby star or stars. Unlike emission nebula the energy from the star is not strong enough to ionize the gas and so it does not emit it's own light.