Primary sources can be described as those sources that are closest to the origin of the information. They contain raw information and thus, must be interpreted by researchers. Secondary sources are closely related to primary sources and often interpret them.
You might cite statistics gathered by law enforcement authorities and organizations like the Highway Traffic Safety Administration in an article describing the elements known to reduce motor vehicle accidents (primary source). You could also rely on secondary sources like books about improvements in vehicle safety and articles about statistics on auto accidents (secondary source).
A paper disputing widely held biographical details about a historical figure in which you include quotes from their journals (primary source) and compare and contrast your findings.