Plyometrics: Also known as jump training, plyometrics is a form of conditioning aimed at created controlled impact and maximum power and is used primarily by athletes.
This method involves stretching the muscles prior to contracting them and done correctly it strengthens muscles, increases vertical jump and decreases impact forces on the joints. Consciously practicing the technique of landing from a jump in a controlled environment teaches your body how to "land" and your muscles memory will kick in when you cannot focus all your attention on your technique (because you are chasing a rebound or opposing team member). It is great for sports that involve jumping like basketball, volleyball and tennis, but is also really effective for training to ski moguls (especially jumping sideways onto or a "step"). Upper body plyometrics are used to condition for boxing (pack a power PUNCH) and pitching in baseball.
Designed to increase muscular power and explosiveness, plyometrics are not, however, for those who are in poor condition or have an orthopedic condition. Since so much jumping is involved plyometrics can be really hard on the knee joints.
Check out Force Sport Fitness Advisor for some animated explanations of plyo exercises.