A battery pack can be attached to your camera allowing you to bypass the battery used in the camera. Many packs have switches on the back enabling you to choose either the camera's batteries or the pack's batteries. Battery packs are ideal for digital cameras and some models will also let you power your laptop computer and cell phone, etc. These items are usually quicker than your camera's batteries.
Battery packs are often used for mounted flash units. A battery pack will greatly improve your flash recycle time and increase the amount of flashes before you will need to replace the batteries. Battery packs can hold various numbers and sizes of batteries.
You can also get rechargeable battery packs, and cases for them with shoulder straps. They generally range in price from about $40 to several hundred.