Shape of the ski - skinny in the middle, fat at the ends.
The more exaggerated the shape, the greater the sidecut.
The greater the sidecut, the smaller a turn the ski will try to make if you just put it on edge and ride it (carving). Keep in mind that you still control the size of the turn, the ski's sidecut just makes it easier to do a certain sized turn.
A slalom ski for making really tight turns will have a very exaggerated shape/side cut, because it is designed to make quick small turns. All-mountain skis have a less exagerated shape since they are meant for all shapes and sizes of turns. Hardcore fat powder skis have very little sidecut because they aren't meant for carving, and need extra ski width under the boot.