Blazars are sub-class of the radio-loud Active Galactic Nuclei (AGN), which emits highly luminous, polarized and non-thermal radiation covering the entire electromagnetic spectrum from a highly compact center (i.e., SMBH). The blazars with higher luminosity and jet angle are known as FSRQs (Flat spectrum radio quasars), while the BL Lac objects show comparatively low luminosity and smaller jet angle (in both cases the jet angle is < 10 degree). Moreover, the SED synchrotron peak of FSRQs lies in the radio to optical range but for BL Lacs the synchrotron hump peaks in UV/X-rays. One of the most interesting phenomenon that blazars show is the multi-frequency flux variation on diverse timescales.

The aim of this thesis is to explore the twisted emission mechanisms involved in the local environment of the SMBH that resides at the center of blazars and to study variability properties in multi-wavelength scenario. We have undertaken two different approach to understand the blazar emission; (1) temporal study in multi-frequency domain and (2) X-ray spectral study. The work presented in this thesis is based on the observations made by XMM-Newton observatory. As part of the first approach, we have studied variability on Intraday & Long (years) timescales and correlated variability between optical, UV & X-ray bands for the FSRQ 3C 273 and found that two electron populations with different Lorentz factors are involved in low and high energy emissions. A hardness ratio analysis in the X-ray regime indicates that the particle acceleration mechanism dominates the cooling mechanism during most of the time. An extensive study with possible correlations, variability timescales and QPOs in a sample of ISP blazars in their outburst state, with the most efficient techniques like SF, ACF, DCF & PSD to understand the complex emission from the relativistic jet and the jet structure has also carried out. We have studied the relation between synchrotron peak of multi-band SED and the X-ray IDV for a sample of a dozen of blazars (includes 10 LSPs and 2 ISPs). Here we have concluded that the highest energy electron available for syncrotron emission will have retarded effect on X-ray IDV. X-ray spectral study of the border line blazar 3C 273 in the energy range 2.5-10 keV, from 2000 through 2015 revealed some new information about the source. A new minimum flux state of 3C 273, even lower than the historical minimum during 2003-2004, was observed in 2015. A harder-when-brighter trend is observed in these long-term multi-epoch observations of 3C 273 for the first time. With simultaneous study of the X-ray continuum and ultraviolet emissions during the quiescent state of the source, we have detect a significant anti-correlation between the X-ray spectral slope and the X-ray to UV flux ratio. From this detection we conclude that the 2.5-10 keV X-ray emission is the result of IC scattering of soft UV seed photons in a thermal corona based above the accretion disc.