A slider is a breaking ball pitch that is thrown harder but with less break than a curveball. A slider is thrown with side spin, rather than the top spin of a curve. This causes a slider to "slide" sideways rather than breaking downward like an overhand curve, and gives the pitch a characteristic appearance of having a red circle caused by the spinning stitching.
A slider is thrown with a grip similar to that of a fastball. Instead of letting the pitch roll off the tips of his fingers, the pitcher gives a twist to his wrist to give the ball sideways spin. The motion of his hand is similar to that of a quarterback throwing a spiral pass.
Historically, the slider was developed much later than the fastball or curve. The first pitchers who threw a pitch called the slider were George Blaeholder and George Uhle in the 1920's and 1930's. The slider became very popular after the Second World War, when pitchers recognized that it was relatively easy to learn and gave them an expanded repertoire.