What is a solar flare ?

NASA’s Solar Dynamics Observatory (SDO) captured this image of an M-class flare on August 17, 2012, at 9:01 PM EDT. The flare, seen on the left (or east) side of the sun from an active region that has not yet been named, and is just rotating into view. The image is colorized in teal, which is typical for showing light in the 131 Angstrom wavelength -- a wavelength particularly good for viewing solar flares.
A solar flare is an intense burst of radiation coming from the release of magnetic energy associated with sunspots. Flares are our solar system’s largest explosive events. They are seen as bright areas on the sun and they can last from minutes to hours. We typically see a solar flare by the photons (or light) it releases, at most every wavelength of the spectrum. The primary ways we monitor flares are in x-rays and optical light. Flares are also sites where particles (electrons, protons, and heavier particles) are accelerated.(Credit to NASA)