Primary sources are original materials (research, objects, documents) from the time period under study. They have not been interpreted, summarized, or analyzed. They were created during the period being studied and documents in some way what is being studied.
These examples from the Lousiana Digital Library are all primary sources:
Draft board summons
Flying Mallard decoy
Snapshot of Zutty Singleton at Vasquez Rocks
Iberville Gazette frontpage
In the sciences, primary sources means original research data, original research articles, or new scientific ideas or discoveries written/prepared by the scientists who performed the experiments or came up with that discovery.
Primary source articles are factual, not interpretive. They are written by the scientists who performed the research.
Importantly, articles that are primary sources in the sciences generally follow IMRD format--Introduction, Methods, Results, and Discussion. Sometimes you can even tell from the abstract if it is a primary source based on if it follows IMRD format.
Looking at the structure of the article can help you understand if it is an original research article:
A secondary source is one step removed from the event and analyzes or interprets the primary sources. It can be published or unpublished works.
Here are some examples from LSU Libraries' collections:
A book review
In the sciences, secondary sources are known as review articles. These can be tricky to identify. They also include citations and may look like original research but they are a discussion and evaluation of someone else's research, not a presentation of original research.