Hi Cali,

We think that a solar flare comes when the Sun's magnetic field gets all twisted up. Then somehow it breaks, reconnects and spews out a lot of energy in the form of fast moving particles some of which hit the surface of the Sun and heat it up. The Sun can then give of X-rays and gamma rays.

The biggest flare that we have recorded in the space age happened in Nov. 2003. It was so bright is maxed out out telescopes and other instruments so we don't really know how bright it was. That was part of a series of storms that got dubbed the "Halloween Storms"

More about the Halloween solar storms here: http://www.youtube.com/watch?v=_Y2gv-MoQx4

The main part we see in a solar flare is a brightening on the surface of the Sun, but they are also associated with giant mass ejections (named CMEs for "Coronal Mass Ejection" - the corona is the name for the Sun's outer atmosphere) that move out into the solar system. These often do hit planets. At Earth they can sometimes cause blackouts, problems with radio communications, and loss of satellites. Some of that happened with the Halloween storms. CMEs can also cause the northern and southern lights (also called the aurora) which are described in the video at the URL above.

cheers,