Please notice that the terms primary, secondary etc. resources have a different meaning across academic areas.

In the Sciences and Social sciences, primary sources are the first hand reports of actual research.

Some examples of primary sources:

• Clinical psychologists report the results of a questionaire that they offered to a group of geriatric patients suffering from aphasia
• A neurologist reports the result of a cognitive experiment in which an MRI scanner was used in an innovative way
• A team of chemical engineers reports the result of stress testing a new light weight alloy that is to be used in satellites

The term secondary source is used for 'second hand' reporting in sources like review articles or handbooks. In this case, the writers of the source report about research, but they did not do the research themselves.

In the humanities primary sources are the object of the research.

These are some example of primary historical sources:

• the ancient historian is studying texts by Tacitus or Xenophon
• the early-modern historian is studying the Resolutions of the Trade Council of the Dutch East India Company 
• the modern historian is studying the diaries of Joseph Goebbels

Secondary sources are the publications where researchers publish their results: conference procedures, journal articles, edited volumes and monographs. 

Tertiary sources synthesize the accepted knowledge of the secondary sources and create general overviews of the knowledge in the field. They compose a synthesis of the scattered and detailed research results.

Handbooks and reference works belong to this catagory.

Library catalogues and bibliographic databases are also considered tertiary sources, because they organize the secondary sources with the purpose to help people to find what they need.