The fastball is the most commonly used pitch in baseball and its average velocity is greater than every other pitch. There are different variations of the fastball:

Four-seam Fastball:
A four-seam fastball has the most velocity of all types of fastballs, but usually has the least movement. It is the most basic pitch in every arsenal and the foundation of ever pitcher’s success.

Normally, pitchers place their index finger and middle finger across the seams — where the Rawlings sign is located; i.e. the horseshoe — with medium to light grip pressure. The distance between the fingers should fall naturally.  The thumb should rest between those two fingers with a clinched ring, and the pinky finger should be rested right on the ball with as little pressure as possible.

Ideally, the amount of pressure should rest entirely on the fingertips and pitchers should be able to tuck a few fingers between the ball and their palm.  Four-seamers are intended for minimum friction; the decreased friction means less drag and induced spin, which increases the speed out of the hand.

Two-seam Fastball:
Two-seam fastballs are thrown with slightly less velocity than a four-seamer (generally 2-4 mph less), but have more movement and sink.

Pitchers throwing two-seamers must find a balance between higher speeds and movement.  It should not have the egg-like pressure with fingertips and should not have any palm exposure, similar to a change up.

The two-seam grip is two based fingers with the thumb underneath the ball in between the index and middle fingers, resting at the top of the ball.  Both fingers should ride with the seams and should be applying pressure over each seam.  The most amount of pressure should be at the fingertips and slightly past the knuckles.  Increasing or decreasing the pressure in one finger over the other helps slightly differentiate the movement of the ball.

Typically, there will be more sink if a pitcher applies more pressure over the middle finger rather than the index finger.  Some guys will even pronate the hand slightly upon release to exacerbate the sinking action.

Cutter or Cut Fastball:
A cutter or cut fastball is a slight variation of the four-seam fastball, but moves almost like a slider.

The cutter should be gripped similarly to the two-seam fastball, but both fingers should hug the outside thread of the inside seam.  The finger pressure should be tight, with the middle finger getting the most amount of pressure.  The thumb should be on the opposite thread underneath the ball with slightly tight pressure, and should be pointing out.  This type of pressure gives the ball more of a slider type feel.  The ball is thrown by spinning it off the middle and index fingers, but the wrist action should mirror a fastball.

The result should be a straight path that veers away from the hand side at the very last moment.  Simple tweaks in finger pressure  and the amount of release your fingers use dictate the amount of slicing action on the ball.

If a pitcher has strong and long fingers, this pitch can have nasty lateral movement and will be almost undetectable until it is too late for the batter to pick it up.  It is the reason why pitchers like Mariano Rivera can make a long career with the cutter alone.

Split-finger or Splitter:
A split-fingered fastball is thrown like a fastball and looks like one before it gets to the plate, where it drops off the table and often has changeup like dip.