Why does a baseball curve?

In baseball, a slider, or nickel curve, is a pitch halfway between a curveball and a fastball, with less break but more speed than the curve. It tends to drop less and move toward or away from the batter more than a curve. The extra speed can fool the hitter into thinking it is a fastball until it's too late. Some pitchers also use a cut fastball (or cutter), which is one step closer than the slider to the fastball on the spectrum between fastballs and curves. A pitch that has movement similar to both a slider and a curveball is sometimes called a slurve.