"Primary sources are materials in a variety of formats, created at the time under study, that serve as original evidence documenting a time period, event, people, idea, or work." -- Guidelines for Primary Source Literacy
A variety of original materials:
A variety of formats, both physical or digital:
Primary sources are original materials that have not been filtered through interpretation or evaluation by a second party.
Secondary Sources are one step removed from primary sources, though they often quote or otherwise use primary sources. They can cover the same topic, but add some interpretation and analysis. Secondary sources can include:
Adding a layer of confusion:
Whether something is a primary or secondary source will often depend upon the topic and its use.
A biology textbook would be considered a secondary source in the field of biology, since it describes and interprets the science but makes no original contribution to it.
If the topic is "science education and the history of textbooks", textbooks could be used as primary sources to analyze how they have changed over time.