Action Preset Camera Mode

Almost every camera on the market today has an "action" mode. Many of these cameras use an icon of a running man to designate this preset. The purpose of this mode is to make is to tell the camera to default its settings toward capturing images faster.
This is done by:
1.Increasing Shutter Speed
2.Increasing Film Speed

While increasing the shutter speed will help to "freeze" the action (stop moving subjects in mid-motion), it also decreases the amount of light striking the film surface. Increasing the film speed helps to offset this light loss by increasing the sensitivity of the film (making the camera need less light to record an image). However, the increase in film sensitivity is often not enough to make up for the loss of light from the faster shutter speed and this can result in underexposed images. The other control for increasing the light striking the film is aperture. Unfortunately, the aperture may not be able to open wide enough in all situations to avoid a dark image. And even in cases when it does open wide enough, the depth of field is reduced drastically so that only a tiny portion of your image is in focus.

This does not mean that action mode is a bad thing. It simply means that you must understand the limits of your camera before shooting a subject. Most camera action modes are best suited for brightly lit action shots such as daytime ball games or moving animals.

Some cameras also switch to continuous focusing when in action mode. This means that the camera constantly tracks the subject and adjusts focus until the moment the shutter button is pressed. This makes it much easier to keep a moving object in focus while composing an image.

Source : http://photography.about.com/