An emission nebula is an area of gas and dust, left over from a supernova (exploding star at the end of its lifetime). The gas in an emission nebula is heated/excited by radiation from nearby stars and does actually glow, giving off its own light. Common gasses here that can be detected by amateurs are ionized Hydrogen, Oxygen, Sulfur, and Nitrogen. The colors most often given off by these gases are reddish and blue/cyan. These can make beautiful full color images, but also false color by combining the light from these elements in different ways in post processing.
Sharpless 308, or the Dolphin Head Nebula