Flares in general are x-ray eruptions on the sun. If one looks at the scale
of the x-ray
flux-plot of the GOES satellite, the sections A, B, C, M and X become apparent.
These are just arbitrary terms for the different sections. The sections are
certainly not arbitrary, though, they are segments of the y-axis. This axis
represents the measured power of the x-ray in Watt per squarmeter. The values
are plotted on a logarithmic scale, wherefore the step from A to B corresponds
to a factor of 10 or just an order of magnitude. This is analogous for all the
other regions as well.
During quiet times the the solar activity, represented by the x-ray flux during
small flares, is in the regions A and B. This is something like a background
activity. With a more complex sunspots present on the solar disc C-class flares
are possible and also small M-class flares can happen. If an extremly chaotic
sunspot appears, the highest section of x-ray flux can be reached, the X-class
level. So far the strongest solar flare reach the class X-30, but this was only
extrapolated as the sensors were heavily saturated.
By the way, one can make its picture of such an X-class producing sunspot, by
puting a normal bar magnet together like a clew. This sounds rather drastic,
but the necessary energy for such big flares has to be produced in some place.