a true slider can be considered as a cutter. Major league cutter usually break only 6 inches across and inches down which is more than enough. I good slider will look and act like a fastball but have a late cutting dropping action across the plate.
A cutter is normal about2 or 3 inches of break jus so it can dive out of the way of the hitters sweet spot or jam him on the hands.
to have an excellent slider you must be able to locate it wherever whenever you like.
to throw one you must stay on top of the ball then cut through it during release.