ok..... shocks are basically a sealed hydaulic cylinder full of light weight oil. At the end of the shaft in the oil is a piston. this piston has holes in it and either a stack of different diameter and thickness shims (most shocks) or spring loaded check balls and shims (works) those shims and check balls control the anmount of oil that bypasses the piston and are arranged to control the oil very procisely and progressivly, flowin more when pushed easily and less when compressed forcefully.
this is what is refered to as the valving.
The aftermarket shocks have much better and more specific valving. they are built from better components. they also have spring rates specifically selected. Theyare also nitrogen pressurized, this keeps the fluid from bioling and aerating when they get hot.
basically if you think of shocks as cars, they all drive down the road, but stock shocks are kia rio's and aftermarket are cadillac escalades..