From the projectile motion unit of physics class, i learned that a smooth ball has forces acting on it as it passes through the air: Gravity and the force of friction, air resistance. A baseball, on the otherhand has laces so the forces act a bit different. The baseball still has the force of gravity acting on it, although because of the laces the air resistance is more affective known as "Drag".
Drag has a very lage influece on a baseball. if the ball were to be thrown or hit in a vacume, there would be no drag, so the path of the ball would be a perfect parobolic shape. Although as all baseball players know, there is not a perfect parobolic shape. The drag slows down the ball in the air and, as seen in the graph above, the ball slows down and finishes its path earlier than predicted. Drag also affects a pitcher. When pitching the ball, drag can slow the ball 8-10 miles per hour in the 60 feet it travels.
On a spinning baseball, there is also a force acting on it known as the "Magnus" force. The magnus force describes the about of airflow past a spinning ball. For example, many people have questioned if a fastball can rise. And the answer to that is yes, only if the upward magnus force is greater than the force of gravity on the ball. Although this may not be possible for a human to preform. If you were to use a foam ball, and apply backspin on the ball, or an upward magnus force, then the ball will significantly rise. This video at 42:20 minutes explains and shows most of what i have said.
Finally, i have always been very curious to why a knuckleball dances. And for many years, even physicists have been baffled to why the knuckleball moves the way it does. This website, http://accessscience.com/studycenter.aspx?main=17&questionID=5579, explains well why the knuckleball dances. What i have found is that this occures because it doesnt spin as much. Because of the lack of spin, the laces create air pockets of low preasure around the ball. When the air rushes into the pockets, it pushes the ball into different directions making it "dance".