An emission nebula
is a cloud of ionized gas. The most common source for ionizations are high energy photons emitted from a nearby star. These can be among some of the most colorful objects in the sky.Reflection Nebulae
are clouds of dust which are simply reflecting the light of a nearby star or stars. The nearby star or stars are not hot enough to cause ionization in the gas of the nebula like in emission nebulae but are bright enough to give sufficient scattering to make the dust visible.
Besides the luminous bright nebulae, there are dark or absorption nebulae
which can be seen because they obscure, or absorb, the light coming from stars or bright nebulae behind them. A famous example is the Horsehead Nebula