Blazars are the most extreme type of active galactic nuclei (AGN)
known. They are characterized by a featureless continuum, high polarization,
and variability at all wavelengths and time-scales. These characteristics
are consistent with a supermassive black hole accreting matter at the heart
of the host galaxy. Various tools can be employed to study blazar
variability, including structure function analysis, power density spectra
(PDS), and construction of simple light curves. By transforming observed
measurements into the rest frame of the object, the intrinsic variability
properties can be revealed. These variability characteristics of
blazars, as seen in their rest frame, will be discussed in addition to
their general characteristics and classification schemes.