Blazars are active galactic nuclei (AGN) sources that display strong, compact, flat-spectrum radio emission; and which also show continuum domination of the optical emission, and/or significant optical polarization, and/or strong gamma-ray emission, and/or significant changes in optical flux on short time scales.
These observational properties are believed to result from emission by material in a relativistic jet which is directed within about 10 degrees of the line of sight. The blazar class includes objects classified as BL Lacertae type objects, high polarization quasars (HPQ), and OVV quasars.
Flaring lazars sometimes display an apparent gamma-ray luminosity as much as one hundred times larger than that at all other wavelength. Variability of the gamma-ray flux from some blazars on a time-scale as short as 4 hours (Mattox et al. 1997) implies that the gamma-rays are emitted from a compact region. The gamma-ray emission is not understood. It may offer fresh insight into the blazar phenomenon, and useful diagnostics.
Here is information on this topic from imagine.gsfc.nasa.gov.