what is the difference between shell and extrude cut ?

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.