Visualizations are not just graphs or charts, but tools which "can be used to highlight the important aspects of a data set or results." At their best, a great visualization can capture complex relationships and enormous volumes of information and distill it into a single image.
If, however, you haven't worked with charts or visualizations all your adult life, it can sometimes be confusing as to what is communicated. Also, there are charts or visualizations which are specific to answering specific types of questions. This website, Vizpedia, gives you an easy "glance" through of the various chart types and explains a little bit about them. There are a number of sites which focus on good design for the infographic or the chart. The Information Is Beautiful Awards site is one of them. That site, however, does not have as much detail as the others about what data source is, or how it was created. Tableau (which is our reporting tool) has the Viz of the Day and a Gallery. Many of the visualizations there can be downloaded along with the dataset. That means you can learn how the visualization was created, as well as explore the actual dataset itself.