Think about what your skeleton is doing when you move. When you move your arm up and down, one end of your arm is pivoting in place while the other end is being moved through third dimensional space. Motion capture uses this idea to be able record a body's movement. A performer will wear a specially designed suit that has 20 to 30 reflective dots and balls on it. These dots are strategically placed on the suit and are designed to either reflect light or to emit infra-red light. When the performer moves, so do the dots. Several cameras record the light from the dots and balls in order to attain various perspectives of the movement. This is necessary because the realm of a computer animated character is three dimensional space.
Hold your finger in front of your face. Slowly begin to move your finger from one side of your face to the other, letting your eyes follow the movement. This is tracking, which is the next step in motion capture. After the performer has finished his movement, the information the camera recorded is then post processed. This involves a computer tracking each individual ball and dot as it moves. It can take as long as two minutes to track one second of movement.
Pinch a bit of your shirt between your fingers, and lift up your hand. What happened? Part of the shirt moved. This is the basic idea behind the next step in motion capture. Each ball and dot is matched up to an equally corresponding location on a computer generated character. If each moving dot is pinned to the same location on the character as it was on the performer, what will happen? It will move. This movement is animation.