A cleanup hitter in baseball is a batter who has a knack for driving in runners. He is best used in situations where multiple runners on base, because he can "clean up" the basepaths with a big hit.
A team's best cleanup hitter will usually bat fourth. Most teams will put batters who can get on base in the first, second, and third positions, to set up their cleanup hitter. A second cleanup position is the eight spot, usually reserved for a high power guy who can't hit for average.
During the dead ball era of baseball, the cleanup spot was usually just reserved for a player who could make contact with the ball. Often times the speediest player would be given the fourth spot because he could lay down a drag bunt and beat out the throw. For example, Ty Cobb, who only hit double digit home runs in 2 seasons, cleaned up for the Detroit Tigers, and Rabbit Maranville, the great shortstop for the Boston Braves, batted fourth though he hit only 28 home runs in his 23 season career!
The very first cleanup hitter to earn the title was of course the great Sultan of Swat, Babe Ruth. During his 1920 season in which he hit 54 home runs, he made the switch from second hitter to the fourth spot. The difference was palpable; in 1920 he had 137 runs batted in - and in 1921 he had 171. After Ruth, more teams began looking for a go to guy in the clutch - Jimmie Foxx, Hank Greenberg, Chuck Klein, and Joe Cronin all got their start in baseball for their ability to knock 'em home when it counted.
Perhaps the best cleanup hitter of all time, though, was Hack Wilson of the Chicago Cubs (and Giants and Dodgers). Compared to other cleanup stalwarts such as Ruth, Mickey Mantle, Hank Aaron, and Barry Bonds, this is tough to argue. He was a good home run hitter, compiling 244 dingers. He was a decent hitter, batting .307 in his lifetime. But his real talent was RBIs. He holds the all time record for RBIs in a season with an astounding 191. He finished in the top 3 in the league in RBIs all 6 seasons in which he was a full-time starter. And even more importantly, with runners in scoring position, he batted a full 50 points higher than with the bases empty. Even the great Ruth didn't have this kind of tenacity for driving in his teammates. Unfortunately, Wilson's career was completely eclipsed by his penchant for alcohol, and he was out of the league at 34.
In today's creatine-fueled home run extravaganza, the cleanup hitter's role has diminished, although few teams will put unproven talent there. Today's cleanup hitters, such as Lance Berkman, Sammy Sosa, Bonds, Manny Ramirez, and Alex Rodriguez, still strike fear into a pitcher's heart whenever there's a runner on the bases. And more often than not, that runner will end up crossing home plate - one more run cleaned up. Batter up!