If a slider is a pitch somewhere between a curveball and a fastball, than the cut fastball or "cutter" is a pitch somewhere between a slider and a fastball. The cutter is almost as fast as a fastball but breaks very slightly in the same direction as the slider and curve do. The cutter is generally gripped like a four-seam fastball with the thumb slightly shifted.
What makes a good cutter so devastating is that it is almost impossible to differentiate from a normal fastball. The cutter breaks along the horizontal plane with almost no downward drop, so the batter swings normally only to have the ball run in on his hands and hit the handle of the bat for an almost guarranteed pop up or weak ground out, and often a broken bat to boot.
The modern master of the cut fastball is Mariano Rivera of the New York Yankees.