  I present an overview of observational studies of quasars of all types, with
particular emphasis on X-ray observational studies. The presentation is based
on the most popularly accepted unified picture of quasars - collectively
referred to as AGN (active galactic nuclei) in this review. Characteristics of
X-ray spectra and X-ray variability obtained from various X-ray satellites over
the last 5 decades have been presented and discussed. The contribution of AGN
in understanding the cosmic X-ray background is discussed very briefly. Attempt
has been made to provide up-to-date information; however, this is a vast
subject and this presentation is not intended to be comprehensive.
