At their simplest. A shell hollows out the inside of a body to a thickness you specify. The thickness is uniform to the shape of the body the shell is applied to. You can also specify for the shell to break through a face. An extruded cut requires a sketch to create the feature. The sketch can be any closed shape you desire. While I suppose it could mimic a shell feature, you are limited by the body shape being cut. I recommend reading SW help or going to youtube to see some demonstrations.