Retargeting is the process of taking animation developed for one character and using it to drive another character without plotting (or baking) the animation onto either character’s skeleton. You can retarget both keyframe animation and motion data.
Animation can be retargeted from one characterized character to another in the same scene, or you can save takes of animation to be retargeted onto any character.
Using generic biped or quadruped skeletons lets you reuse animation on different characters.
The options that let you quickly retarget animation between characters are found in the Character Controls.
Retargeting keyframe animation
When you retarget keyframe animation, the Reach properties are also retargeted to the target character. This means you can take the precise keyframe animation you have developed for one character and reuse it on another character.
When you select the 
> Edit > Controls > Retarget Rig, keyframes are copied directly from one Control rig to the other, without plotting. The
Character dialog box that appears lets you select how you want the retargeting to be done.
The quality of your animation is preserved and adjustments for any difference in size between the characters are made automatically.
In order to retarget keyframe animation properly, you must have an equivalent setup on both characters. The best way to achieve this is to create identical Control rigs for both the source character (the character you want to retarget from) and the target character (the character you want to retarget to).
