Active Galactic Nuclei (AGNs) are among the most energetic phenomenon in the Universe, which emit extremely luminous, polarized and non-thermal radiation covering the entire electromagnetic spectrum from a highly compact core. Due to their high luminosity, AGNs can be observed out to very high redshift (z > 6), which makes them excellent probe of the early Universe. In this talk I will discuss about AGNs and their importance in astronomy. Besides, I will briefly discuss my research work on spectral and timing variability properties of these objects.