A mouse pad, or "mousing surface," is a surface designed for tracking the motion of a computer mouse. Early mouse pads were literally "pads," which had soft surfaces. The slight cushion allowed the mouse ball to roll smoothly in any direction. Modern mouse pads typically have harder surfaces, designed to track the motion of optical mice.
Mouse pads come in many shapes and sizes. Most are rectangular, though are some are circular or oval shaped. While the majority of mouse pads are less than a foot wide or long, graphic artists and CAD designers sometimes use mouse pads that are much larger. These super-sized mousing surfaces allow them to use a slow, precise mouse speed without needing to lift up the mouse very often. Computer gamers also prefer larger mouse pads, because they offer more desktop real estate for making long, fast motions.
Since optical mice track motion by detecting small changes in the surface below the mouse, the mouse pad's surface affects how accurately the mouse responds to movement. Lighter colored mouse pads typically produce the best results since they reflect the most light. Some highly reflective mouse pads even claim to have "battery saving" capabilities for wireless mice. The improvement in battery life most likely comes from how quickly the mouse enters low-power mode when it stops moving.
Since optical mice work on just about any opaque surface, you typically do not even need a mouse pad to use a mouse. However, a good mouse pad can provide smooth, accurate motion, which is beneficial if cursor accuracy is important for your work. When choosing a mouse pad, make sure you get one large enough for your needs, but not so large that it won't fit on your desk. If possible, choose a mouse pad with a built-in wrist rest or buy a separate wrist rest if necessary. This small ergonomic addition will relieve a lot of strain on your wrist no matter what mouse pad you use.