In the sciences, a primary source:

• is peer-reviewed;
• is published in a scientific journal;
• and contains first-hand reports of research presented by the person or team that did the research.

Primary scholarly references are the gold standard for your background research as a scientist. Secondary scholarly literature—review articles, books, encyclopedias, handbooks, etc.—are useful entry points, but shouldn't be used alone. Follow up on the citations you find in secondary sources to get to the primary scholarly references.

Scientific primary literature is peer reviewed, or refereed, before being published. This enhances the quality and validity of the work. In the peer-review process, two or three specialists in the field read and critically evaluate the work before it can be published. Peer review is a quality-control measure to ensure that the primary literature includes only high quality, valid scientific information. Primary authors may revise and resubmit articles to improve them. Secondary literature is less stringently reviewed.

Modern/Current Newspapers

In humanities disciplines, including history, a primary source:

• Is anything produced by humans during the time period under consideration:
  • Texts, whether published (books, articles, patents, transcripts) or unpublished (diaries, notes, manuscripts);
  • Imaginative work, such as paintings, music, movies, or novels;
  • Visual resources, such as photographs or newsreels;
  • Sound recordings, such as interviews;
  • Material culture, such as scientific instruments, clothing, or garbage dumps.

• Primo is a good starting point for trying to locate primary sources.
   
• For older materials, see if you can find a "scholarly edition" or "documentary history."  These are collections of carefully chosen primary source materials with introductions and annotations provided by scholars.
   
• If you have found a scientific primary source, but are having difficulty understanding it, try to see if you can find a summary in a subject encyclopedia.  For more recent scientific primary sources, you might try to see if a summary or overview has been published in Scientific American, a non-scholarly journal that attempts to translate science for a general audience.