A blazar is a type of Active Galactic Nucleus or "AGN" -- supermassive black holes having millions or billions of times the mass of the Sun, found at the centers of some galaxies. "AGN" is a catch-all term for most types of galaxies that have extremely bright and concentrated emission from their central regions. This light is believed to come from matter in the form of gas, dust, and even whole stars being sucked into the central black hole. As all of this material spirals in, it gets very hot and very bright, much brighter than the stars and other luminous material in the center of the galaxy.
The way an AGN looks to us is dependent upon our line of sight into the central region of the galaxy -- mainly on whether we can see the accretion disk surrounding the black hole or not, and whether relativistic jets are being created by the black hole and are pointed in our direction. If we don't see the central engine of the AGN, and the jets aren't pointed in our direction (or don't exist at all), then it might not appear to vary at all, and might not even be noticed as an AGN without looking carefully at all wavelengths of light. But the blazars are AGN where we not only see the inner accretion disk close to the black hole's event horizon, but we're also looking straight down the beam of the jet. In blazars, we're seeing the most energetic parts of the AGN. Among AGN, the blazars vary on the shortest timescales, and emit radiation across the entire electromagnetic spectrum from radio waves to the highest energy gamma rays.
Blazars are variable at all wavelengths of light at which we've observed them, from radio waves to high-energy gamma rays. Blazars also produce some of the highest energy gamma rays observed in the universe, millions of times more energetic than the gamma rays emitted by radioactive elements, and billions of times more energetic than the X-rays you receive in a doctor's or dentist's office. Only objects with incredible power -- strong gravity, strong magnetic fields, strong radiation pressure, or all three -- can make electromagnetic radiation that energetic, and supermassive black holes certainly fit the bill!
Even more fascinating is that blazars vary on very short timescales, as short as hours or days. This is because when we look at a blazar, we're looking at the very heart of the AGN, right at the black hole itself. Objects can vary on a timescale proportional to their physical size, essentially however long it takes light to cross the entire size of the object. So for blazars to change on timescales of a few hours, they must only be a few light-hours across. For comparison, it takes a ray of light about 11 hours to cross the Solar System from one side of Pluto's orbit to the other. So in a blazar that varies on that timescale, you have to pack millions, billions, or even trillions of solar masses into a volume the size of Pluto's orbit!
In 2003, the AAVSO partnered with the GLAST Telescope Network (GTN, since renamed the "Global Telescope Network") in an Education and Public Outreach project to monitor blazars prior to the launch of GLAST, now known as the Fermi Gamma-Ray Space Telescope. Long term monitoring of a number of blazars helped to establish a baseline of behavior to use for future comparison. The AAVSO International High Energy Network continues to monitor blazars and other high-energy phenomena for these and other projects.
The AAVSO is currently (as of 2008 December) running another campaign on several blazars being monitored by the VERITAS observatory and the XMM-Newton satellite, by request of Dr. Markus Boettcher (Ohio U.). Please see AAVSO Alert Notice #353 for more details!