A switch hitter is a batter who regularly bats both right- and left-handed.
Most batters hit better against opposite-handed pitchers than against same-handed pitchers. In other words, right-handed batters tend to hit better against left-handed pitchers than against right-handed pitchers, and left-handed batters do better against right-handed than left-handed pitchers. This so called platoon advantage drives much of baseball strategy, including the use of pinch hitters and LOOGYs and the decision of some batters to become switch hitters.
In theory, a switch hitter should always have the platoon advantage, since he can always bat opposite-handed from the pitcher. This is true even in the extremely rare case of a switch pitcher, as the current rules prevent the pitcher from changing throwing hands during the course of a plate appearance. In practice, most switch hitters are better hitters from one side of the plate than the other, and some are so much better from one side of the plate than the other that opposing managers will still bring in specialist relievers to force them to hit from their weaker side. For example, J.T. Snow was such a poor hitter batting right-handed that he decided to abandon switch hitting and bat exclusively left-handed.
Interestingly, switch hitters will not always bat with the opposite hand as the pitcher. Some switch hitters find that they fare better batting "wrong-handed" against specific pitchers. For instance, although Tom Glavine threw left-handed, for part of his career he had a reputation of being tougher against right-handed batters than against left-handed batters. Some switch hitters responded by batting left-handed when facing him, against the normal platoon advantage. Some switch hitters would also bat right-handed against Tim Wakefield because they were worried that facing his knuckleball delivery would disrupt their normal left-handed timing against other right-handed pitchers.
All-time MLB leaders among switch hitters:
|SB||Tim Raines Sr.||808||5|
- Joe Posnanski: "The top 10 switch-hitters in baseball right now", mlb.com, June 5, 2018. 
- Andrew Simon: "Here's the greatest switch-hitting lineup of all time", mlb.com, June 17, 2020. 
- Cort Vitty: "Switch-Hit Home Runs: 1920-60", The Baseball Research Journal, SABR, Volume 44, Number 2 (Fall 2015), pp. 10-14.