Boxing and karate both have the goal of damaging the opponent aggressively. Unlike boxing, the Karate fighter keeps his center of gravity low to the ground and depends upon his legs, buttocks, and upright torso to power his strikes. Therefore, the Karate enthusiast does not lean in while punching. Also unlike boxers, Karate enthusiasts are generally taught to strike sensitive weak points in the human body like the throat and places where large nerves or weak bones are vulnerable. Karateists do not usually train wearing protective gear, so they learn to use only one or two knuckles on their fists as striking points making their blows very precise and controlled.
Boxers are limited to punching alone, at least during competitions, but the karate enthusiast will use his feet, elbows, knees, and head as weapons as well as a large variety of hand techniques. A boxer attempts to rain down blows upon the opponent’s head and stomach in order to weaken him and hopefully induce unconsciousness. A karateist will strike more cautiously, waiting for just the right moment to take advantage of some perceived loss of concentration or physical weakness, and then will strike with one to three blows. This difference in tactics is probably influenced by the lack of protective gear in karate. A karate player will attempt to unbalance or take down an opponent by sweeping or damaging his legs. Unlike a boxer, the karateist will follow the opponent to the ground after he has fallen, and will continue to strike him until he is thoroughly defeated. Boxing is practiced exclusively according to sport rules. Some amount of training in karate is practiced without regards to the rules of the sporting component. However, none of that means that someone trained in boxing will not try to kick his opponent if the opportunity presents itself.
[From www.24fightingchickens.com. Original page is here]