Battery Holder - 4xAA to Barrel Jack Connector
Description: This is a simple 4 cell AA battery holder. The 5 inch cable is terminated with a standard 5.5x2.1mm, center positive barrel jack connector.
The connector mates with the barrel jack on the Arduino (among a number of other products) allowing you to easily make your project battery powered.
[Product description and pictures are courtesy of Sparkfun.com]