Data visualization is an umbrella term. It is used for any visual technique that tries to make data easier to understand and interpret by presenting it in a visual format.
The rationale behind data visualization is that certain aspects of data (similarities or differences, trends, interrelationships, patterns) can be more easily identified or clarified using a visual representation than they can in text-based form. From infographics to wordclouds: there are many different forms of data visualization, some better suited for certain sets of data than others.
Take a look at some examples of data visualization from our best practices section.