What is pine tar?
Pine tar is a brown, sticky substance produced by the carbonization of wood. It is readily available for purchase at most sporting goods stores.
How is it used in baseball?
Major League Baseball allows hitters to use pine tar on their bats as long as the brown substance doesn't extend past 18 inches. This is so hitters get a better grip, especially when they're wearing batting gloves, and therefore do not let go of their bats when they take a swing.
What happens when it extends past 18 inches on a hitter's bat?
Ask George Brett.
If it's legal for hitters, why is it illegal for pitchers?
Baseball bans the use of foreign substances such as pine tar (or Vaseline, maple syrup, sandpaper, etc.) to keep pitchers from doctoring baseballs and gaining better spin on their pitches and making them move more violently in the air. However, because pine tar mostly helps pitchers get a better grip in cold weather, most baseball people look the other way as long as the use is discreet.
If pitchers can't use pine tar, what should they use to get a better grip?
A rosin bag -- filled with a white, chalk-like substance -- is always sitting on the back of the pitcher's mound. Rosin essentially is a form of powdered pine tar and is used by pitchers to dry the sweat from their exposed pitching hand. But because it's not a sticky substance, they prefer pine tar to get a better grip on the baseball.