There are two kinds of Nebulae - dark nebulae and bright nebulae. Dark Nebulae are ones which have dark clouds silhouetted against a brighter background.
Bright nebulae are usually bluish and reflect the light of nearby stars.
Nebulae are the material from which stars are born and into which, larger stars blow some of their material when their fuel runs out. When large objects pass through a nebula, the gas and dust are stirred and the birth of a star begins.