There's more than one...

In fastpitch softball there are many pitches a pitcher can throw. The most common pitch to throw is the fastball, which goes straight down the middle of the plate or you can work on location and go inside low or high or outside low or high.
A change-up is one of the most important pitches. If you want the batter to be confused, throw her a change-up. Compared to a fastball the change-up is slower and half the speed of a regular pitch.The way you throw it is crucial to the pitch and it is hard to learn. 

A curve ball is another important pitch if you want the batter to swing at off pitches. When you do this, the ball starts going toward the plate and then curves over towards the outside of the plate if you are right-handed and the inside of the plate if you are left-handed. The pitch similar to this is the screw ball. This pitch is thrown to a left handed batter and is similar to the curve.
A drop ball might be my favorite. It doesn't go directly into the catchers glove, it drops very low. It starts out as a regular pitch until it gets halfway and then it drops. Along with the change up, it is also hard to learn the motion.
A rise ball is one that I don't know how to throw yet. Obviously, like the name of it, it rises. It's the opposite of the drop ball and it is very hard to hit.

Out of all of these pitches, I  know 5 out of 6. I only need to learn the rise ball and then I'll be set!