A nebula is a vast cloud of gas and dust in space, often serving as a nursery for new stars and planetary systems. They can be formed from the remnants of dead stars or as regions where gas and dust are concentrated, leading to star formation. Nebulae come in various types, including emission, reflection, and dark nebulae, each with distinct characteristics and appearances. Notable examples include the Orion Nebula and the Crab Nebula.