Active Galactic nuclei (AGNs) refers to the existence of
energetic phenomena in the nuclei or central regions, of galaxies which cannot be directed to stars. In this talk, I will explain the factors which helps in distinguishing AGNs from stars and what lead
to the Unification model of AGN. A brief overview of the optical spectrum of AGN will also be given. Finally, I will conclude with why are we interested to study such distant objects.