Data visualization is a way to bring meaning to your data through the use of visual tools such as charts and maps. It is essentially "using visual shorthand to present complex [ideas] quickly and clearly."
Want to learn more? Lynda.com provides several comprehensive video tutorials on topics relevant to data visualization including:
The following White Paper published by Perceptual Edge also provides valuable information on graph design and specifically includes "a step-by-step process to present your data clearly and drive your message home."