Blazars are the subclass of active galactic nuclei (AGNs) possessing a relativistic jet that is aligned close 10 degree to the observer’s line of sight. The two classical subclasses of blazars are the BL Lacertae objects (BL Lacs) which have no detectable, or very weak (EW < 5Å), optical emission lines and the flat spectrum radio quasars (FSRQs), which have the usual strong quasar emission lines in their optical spectra. The two broad bumps seen in the broadband spectral energy distributions (SEDs) of blazars indicate two different emission mechanisms, synchrotron and inverse Compton (IC) process. The high energy components of blazar SEDs peak at GeV energies in LBLs and at TeV energies in HBLs. The X-ray emissions of TeV blazars are found to be highly variable at IDV timescales. The Doppler boosted non-thermal emission from the relativistic jets is highly variable at all observed timescales over essentially the entire electromagnetic spectrum. Variability seen over a timescale of less than a day to a few years. I will discuss excess variance test to check its variablity amplitude.