What is an addon handgrip or battery grip?

Many big brand SLR camera producers including Nikon and Canon, now give photographers the option of an addon handgrip / battery grip. The image below demonstrates what a battery grip looks like without the camera attached.

With a digital SLR camera attached, it makes the setup look more squarish than rectangular. Apart from looking larger and more impressive, there are other advantages to using a battery grip.

Advantages of DSLR camera battery grips

• Battery grips make it easier for those with larger hands to grip the camera. In some cases even those with smaller hands will find it useful when holding the camera in a portrait (horizontal) position.
  
  
• Many battery grips nowadays, are designed to hold more than one battery. For example, the battery grip for Canon's EOS 400D (Digital Rebel XT / Kiss), holds two batteries. Many photographers will recall a number of times when they've been midway through a shot, when they find the camera locks up due to a low battery. By the time they've changed the power supply, the shot is long gone. Adding a grip that holds 2 batteries to your setup, will help solve this problem. The camera won't lock up until both batteries are low.
  
  
• Some battery grips have an added advantage of weather protection. Many photographers believe they extend the battery life in cold conditions when normally they run down quicker.