Quasars belong to the most energetic phenomena in the universe. Physical processes at the origin of them can be explained in the framework of the AGN unified scenario. The unified scenario predicts two classes of quasars, obscured and unobscured ones, depending on the orientation of the dusty torus which surrounds the central supermassive black hole. Variability is ubiquitous in AGN, and may be generated by several phenomena that would produce different footprints. Determining the nature and origin of the variability is a key subject to understand the differences between the different types of AGNs.