The Department of Transportation (DOT) and Pipeline and Hazardous Materials Safety Administration (PHMSA) published this helpful chart for understanding hazardous materials markings, labeling and placarding. It contains examples of the markings required for shipping hazardous materials and a guide for when the markings are applicable.
Use the controls at the bottom of the embeded pdf to view all the pages. You can also download a copy using the link below. More information can be found at http://phmsa.dot.gov/hazmatchart 15