are luminous clouds of gas and dust that usually glow because the gas is ionised by UV radiation from nearby stars of high surface temperature. Other less common causes are violent collision with another gas cloud or synchrotron radiation as charged particles spiral around a magnetic field.
Planetary nebulae are shells of gas and dust that are puffed out of ordinary mass stars that are in the process of dying. The nebula glows because it is being bombarded by the UV light emitted from the dying star. With the outer atmosphere of the star blown away, what remains is a hot core whose temperature is typically 100,000K - hence the UV! The star will turn into a white dwarf that slowly fades, becoming a so called black dwarf. Our own star, the sun, wil end its life this way in about 5 billion years time.
Reflection nebula is a cloud of gas and dust that reflects or scatters starlight.
Open clusters are groups of stars that formed together in the spiral arms of a galaxy.
Globular clusters are roughly spherical groups of old stars in the halo of a galaxy. They can contain a million stars with a very high concentration in their cores.