As research is created and disseminated, it moves from being a primary source → → toward being → → a tertiary source.
The earlier in the process (and closer to the actual science being done) the more likely you are dealing with a primary sources.
When you start to research a topic (before you do any science), you go the opposite direction ← ←. You may start with a tertiary source, like Wikipedia or library database and move toward the original work or primary research.
Primary Sources in the Natural Sciences
This video will highlight the differences between primary sources, secondary sources, and review articles in the Natural Sciences. (Time 4:34)