A battery grip is an accessory for an SLR/DSLR (and occasionally other camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s), which allows the camera to hold multiple batteries to extend the battery life of the camera, and adds a vertical grip with an extra shutter release (and other controls), facilitating the shooting of portrait photography

Portrait photography

Portrait photography or portraiture is the capture by means of photography of the likeness of a person or a small group of people , in which the face and expression is predominant. The objective is to display the likeness, personality, and even the mood of the subject...

.

It usually attaches to the camera body through the camera's own battery compartment and provides a cassette to hold additional batteries

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

to increase the battery life for the camera. Most battery grips also come with a second cassette allowing the photographer to shoot using multiple AA batteries.

Battery grips are usually designed to fit only one or a few specific camera models, since they must match the body's shape, connectors, and power requirements.