During sport, sledging refers to the practice of talking to your opponent with the aim of putting them off their game.
Sledging takes many forms, and is limited only by imagination, wit, or good taste. Most sledging consists of simple insults aimed at either the opponent's ability, character, spouse, or family. Sledging can be venomous, light-hearted, or anything in between. In some sports it is rife, while in others it is rare. Some consider it unsportsmanlike, others see it as harmless good fun, and others as a serious tactic to be employed in the service of winning.
Here are two famous examples from the game of cricket. The first is so cruel as to almost be beyond belief, while the second has unfortunately been confirmed as apocryphal.
Shane Warne to Chris Cairns - Chris Cairns' sister had recently died in a train crash, and when Chris was walking onto the field to begin his innings, Shane walked alongside him making "choo choo" train noises. If this one is true, then it's the no-contest winner for the cruelest sledge ever.
Steve Waugh to Herschelle Gibbs - In the 1999 Cricket World Cup, Australian captain Steve Waugh was playing a rescue innings in a must-win match against South Africa. With the Australian innings in disarray, he was the last slim hope for an Australian victory. On 56 he played the ball straight to Herschelle Gibbs, one of the best fielders in the world. Gibbs caught it and simultaneously threw the ball into the air in celebration, except that his celebratory throw occurred a fraction before he actually had the ball in his hand, resulting in neither catch nor celebratory throw. In other words, he dropped a catch in the most embarrassing way possible.
At the end of the over, Steve Waugh walked over to Gibbs and told him, "You just dropped the World Cup, son". Waugh then proceeded to play a match-winning innings of 120, single-handedly rescuing his side, which then went on to win the World Cup after knocking out the South Africans in the semifinals.