Charts in PowerPoint can be customized by changing the appearance of various chart elements. The legend is also a chart element and typically it is a box or area that codes via color or pattern all the series in a chart. The legend can be formatted to be placed at various locations within the chart area, and you can hide it altogether.
Learn more about formatting the chart legend in PowerPoint 2003.
Learn more about formatting the chart legend in PowerPoint 2007.
Learn more about formatting the chart legend in PowerPoint 2010.

