Two-seam fastball

Your first two fingers lay on the narrow part of the seams, with your thumb, the third point in the triangle, also on the seam below.  The wrist basically goes straight through in a downward motion, always with a follow-through.  You want the ball to have a backspin caused by your two fingers rolling off the seam on release.  Your thumb is the last in your line of mechanics to control the direction of the pitch.  Don't underestimate it and overlook it.  With the curves of the seam turning straight, the ball should drop sooner than a four-seam.  Remember, the lower the arm action, the more the ball moves.

Four-seam fastball

We say 2-seam and 4-seam, but actually baseballs only have one seam.  It's the 4 loops or curves that gives us the expression.  With the curves of the four-seam turning on the outside of the ball, it should lift and thereby be faster.  Remember there's more speed up, and more movement down.  Whether you are right-handed or left-handed, your middle finger (the longest) should be on the wider side of the curve.  The grip itself, with your fingers across more turns of the seam, will be stronger, and that should result in increased speed.

Palm or circle change

The palm, or circle, change gets it's name from the grip and the resulting difference in speed.  It's called the palm change because the ball sits all the way back in the palm.  It's called the circle change because of the ok circle made by the thumb and index finger.  It's called a change because, although it's thrown with the same mechanics and speed as the fastball, it should move a few miles an hour slower.  There are many different versions of a changeup pitch.  You are encouraged to experiment to find what works for you.   

Curve

Curveball

A curveball is supposed to fool the batter into swinging where there is no ball.  The ball is supposed to move out of the way of the bat.  There's nothing more picturesque for the pitcher and his fans to see a batter who is waiting for the curveball fall flat on her face when she misses it.  With the ball griped as illustrated, the wrist makes a J-turn on the delivery, resulting in a palm-up follow-through. 

Sinker

The sinker is essentially a curveball that moves down as it reaches the plate.  There is more rotation put on the ball because the thumb also flips the seam up on release.  There is more pressure on the wrist with a sinker than there is with a typical curveball.  

Slider

Football slider

There are many sliders and cut-fastballs that are supposed to miss the bat, and the football slider seems the easiest to master.  The grip is essentially the same as a curveball, but the release is different.  It should be thrown like a football by the hand, but with the full baseball motion of the rest of the body.  There are several variations to the release, but the one illustrated gets results and is easy on the arm.  

Forkball/Splitter

The forkball is not a splitter, but it is very similar.  The grip shown is a splitter grip; the forkball would have the narrow part of the seam between the fingers.  This is the kind of pitch that often stays in a pitcher's practice arsenal for years before ever making an appearance in a game.  It's difficult to throw with control and confidence, and  it's rewards are marginal.  That is, until it is developed.  This is the kind of pitch a strike-out pitcher needs in his arsenal to fan the good hitters that either catch up to his fastball or are using the curveball for batting practice.

Knuckleball

Knuckler 1

Most pitching coaches will tell you that knuckleballs are for knuckleheads.  This may be true of those pitchers that use a knuckleball just to use it.  But if a pitcher takes the time to develop this pitch to use in games, he will see why hitters shy away from even swinging at it.  The pitch is delivered by flicking the fingers forward, and the ball comes to the batter with little or no rotation.

Knuckler 2

So if knuckleballs are for knuckleheads, why do we have two.  Two knuckleheads are better than one?  Well, as you can see, there are two different grips.  This is the one I prefer, but my son likes the one above.  I use it in playing catch, including throwing high pops (try it; it's weird to catch too), and he uses it while pitching.  Still, usually in practice, and rarely in a game.