Short form: Club class allows a fairly narrow range of handicaps,
and also can fly Assigned Tasks. Sports class allows anything to fly
but can only fly Area and MAT tasks. There's not a lot of Club class
events in the US -- mainly just the Nationals. Occasionally there's
a regional that will have club class. Outside the US I think Club
class is pretty much all you find.

Long form, there's a lot of ink expended on the subject which you can
find readily. There's been threads here; there's the US rules; there's
the FAI rules; there's websites...

Matt