Stiffness refers to the amount a ski will bend when force is applied to the ski.
Generally the softer constructions are better for softer snow, and more maneuverable and easy to ski at lower speeds. The stiffer constructions are more stable at higher speeds, better on harder snow, but a bit harder to turn. A Medium to Medium / Stiff ski is often the best choice for all around riding.
Soft
Medium / Soft
Medium
Medium / Stiff
Stiff

