http://www.hubblesite.org/gallery/album/nebula/pr2002024a/
The word “nebula” is derived from the Latin word for “clouds”. Indeed, a nebula is a cosmic cloud of gas and dust floating in space. A nebula is a huge, diffuse cloud of gas and dust in intergalactic space. The gas in nebulae (the plural of nebula) is mostly hydrogen gas (H2).
More than one nebula are called nebulae. Nebulae are the basic building blocks of the universe. They contain the elements from which stars and solar systems are built.
They glow with rich colours of reds, blues and greens with swirls of light. Stars inside these clouds of gas cause them to glow in colours.
Most nebulae are composed of about 90% hydrogen, 10% helium, and 0.1% heavy elements such as carbon, nitrogen, magnesium, potassium, calcium, iron. These clouds of matter are also quite large. In fact, they are among the largest objects in the galaxy.