A nebula is a large cloud of dust and gas in space, often appearing in different colors due to the elements within. Nebulas form during both the birth and death of stars, with the Eagle Nebula showcased as an example. The document then lists several notable nebulas like the Boomerang Nebula, Horsehead Nebula, Cat's Eye Nebula, Egg Nebula, Carina Nebula, and Crab Nebula.