Deep-sky targets fall into a few families. Emission nebulae are clouds of hydrogen/oxygen/sulfur gas glowing at specific wavelengths (great for narrowband). Reflection nebulae scatter blue starlight. Galaxies are distant island universes — small and faint, rewarding long focal lengths. Star clusters (open and globular) are bright and beginner-friendly. Supernova remnants and planetary nebulae are the colorful debris of dying stars. Brightness (magnitude) and apparent size determine the exposure and focal length you need.