In baseball, a pitch is the act of throwing a baseball toward home plate to start a play.
Pitchers throw a variety of pitches, each of which has a slightly different velocity, trajectory, movement, hand position, wrist position and/or arm angle. These variations are introduced to confuse the batter in various ways, and ultimately aid the defensive team in getting the batter or baserunners out. To obtain variety, and therefore enhance defensive baseball strategy, the pitcher manipulates the grip on the ball at the point of release. The selection of which pitch to use can depend on a wide variety of factors including the type of hitter who is being faced; whether there are any base runners; how many outs have been made in the inning; and the current score. 
The fastball is the most common pitch in baseball, and most pitchers have some form of a fastball in their arsenal. The 4-seam fastball is the most common pitch when referring to Fastball. It is the most basic pitch for every pitcher and is also what most players including fielder use to throw when playing. The 4-seam fastball is also called the Straight as the 4-seam spin will help the pitch cuts through the air with lower resistance resulting in higher velocity of the pitch with no movement.
There are fastballs variances that have almost comparable speed to that of a 4-seam but also has (most often late) breaking actions of a breaking pitch. These pitches are known as Breaking Fastballs and listed below are the major breaking fastballs used by pitchers:
Well-thrown breaking balls have movement, usually sideways or downward. A ball "moves" due to the changes in the pressure of the air surrounding the ball as a result of the kind of pitch thrown. Therefore, the ball keeps "moving" in the path of least resistance, which constantly changes. The goal is usually to make the ball difficult to hit or confusing to batters. Most breaking balls have slower speed than Fastballs and can be used as off-speed pitches. The most common breaking pitches are:
The Changeup is the staple off-speed pitch, usually thrown to look like a fastball but arriving much slower to the plate. Its reduced speed coupled with its deceptive delivery is meant to confuse the batter's timing. It is meant to be thrown the same as a fastball, but simply farther back in the hand, which makes it release from the hand slower but still retaining the look of a fastball. If thrown correctly, the changeup will confuse the batter because the human eye cannot discern that the ball is coming significantly slower until it is around 30 feet from the plate.
There are also various types of changeups that pitchers can employ including the conventional Circle Changeup, Palmball and Vulcan Changeup.
Other pitches which are or have been used in baseball are: