Kara Cancel
Fighting games have a bit of leniency when it comes to inputs and will autocorrect when executing complex motions like special attacks and throws if an "incorrect" input is pressed 1 frame before a "correct" input. If I press the kick button 1 frame before a throw input, the game assumes I was going for a throw but I accidentally pressed the kick button. A kick will startup for 1 frame then the game autocorrects by canceling the kick and starting the throw. This is a kara cancel.
Almost every normal attack can be kara canceled but it's not beneficial for 99% of attacks. Kara canceling is primarily used for adding range to special attacks and throws. This is possible with an attack that moves the character forward on frame 1 before it is kara canceled. In this example Kairi's kick starts up before it kara cancels into his special attack uppercut.
Kara canceling utilization is very character dependent. For some characters, kara canceling is a vital part of their playstyle while other characters may not have a single attack that is useful for kara canceling.
Other than making an attack startup 1 frame slower, the only drawback to kara canceling is execution. Some kara cancels are extremely difficult to execute and failing is very unsafe. Some kara cancels are difficult to execute and the reward is minimal.