In order to understand the mechanics of animation we have to understand how movement is recorded by a live action camera. The movement of an actor can be recorded through a traditional cinema camera or digital cinema camera but regardless either way the camera is capturing those images at 24 frames per second. When the 24 frames are play back quickly the images appear as motion. You take any video clip and open it up in quicktime and then pause the video. Then you can use the arrows on the keyboard to move forward frame by frame on that particular video clip and them you can see the individual pictures with in the video.
In traditional hand drawn animation instead of taking pictures of an actor artist draw the individual frames for each shot of the movie. An animator will sit at a light table and that will allows them to see several different frames at once so they can keep track of the movement of their character. When these images are played back a regular speed if will give the illusion of movement.
For a better understanding of the traditional process an animation grab a stack of post it notes and take a pencil or a pen and create one drawing on each post it, then flip through the stack. The advantage of using a computer for animation is you can select an object and set a key frame at the beginning of the clip and then move that object a set distance and set another key frame at the end of the clip. Then the computer will go in and place that object on each of the frames that are need between the beginning and end to complete the animation for you.Even with the help of a computer animation is an extremely slow process. In order to figure out how many frames we need for a feature length movie which is around 90 minutes you have to 24 frames per second and multiple it by 60 seconds which gives you 1440 frames per minute.Then take the 1440 and multiple that by 90 for a 90 minute movie and you end up with 129,600 frames. This shows how much work it takes to make an animated film.