Rotoscoping was invented in 1915 by animator Max Fleischer. The technique allowed Animators to trace films and create life like animations.
Disney would later copy this technique and use it in films such as Snow White, Peter Pan and Cinderella.
In recent times Richard Linklater has created two films Scanner Darkly and Waking Life using digital techniques that are similar to the rotoscoping idea.
Use Frame by Frame animation to rotoscope (trace) a video.
Experiment with Photoshop Brushes, colors and effects.
Here is an example of a skateboard video and the resulting Rotoscope.
Add a Layer Mask to a video.
Use Frame by Frame animation to Layer Mask a portion of each frame and repeat and adjust the mask in each consecutive frame. Use the resulting video to make a composite video. Here is an example of the skate video being masked and then composited against a shark video.