I believe it is mostly referring to the stored energy to peak ratio (SEP) or the energy stored in the limbs of the bow relative to the peak weight. A typical "round" wheel will have an SEP of around 1.0 which means that if the amount of energy stored at full draw is 50 ft/lbs then the peak weight will be 50 pounds. This would be called "soft". As you increase the severity of the camming and store more and more energy in the bow realtive to the peak the SEP ratio goes up. A ratio of 1.4 is pretty high and would be referred to as a "hard" cam. At 50 pounds peak weight a 1.4 SEP bow would store 50 x 1.4 = 70 ft/lbs of energy. It's generally true (though not absolutely true) that as you increase the SEP you will also increase the slope of the "wall" as NCYankee was referring. Higher SEP's almost always mean a more abrupt wall. As SEP increases the shape of the force draw curve more closely resembles a square rather than a smoothcurve, is harder to pull back and doesn't feel as smooth. Hence the expression soft and hard cam.