Q. What are the differences between primary, secondary, and tertiary sources?

Wikipedia actually has pretty good overviews for each of these source types via their pages:

• A primary source 
• A secondary source
• A tertiary source 

In academia, we most often contextualize their differences in terms of articles....scholarly articles are published in scholarly/academic journals, and they are usually based on some actual research (case study, experiment, trial, etc.). They are usually written by experts in the field for other experts in that or a related field, or people who are studying to become experts (scholars) in that or a related field. And in many disciplines, this research (and even the articles themselves) are considered primary sources. Textbooks, on the other hand, would be considered scholarly, but they would be secondary sources, not primary.

What constitutes a primary source is often different depending on the field you're in. An example in biology might be a lab report (a write-up by a scientist about the results of an experiment). In the social sciences, a primary source might be a diary, a letter or even a newspaper article written by somebody who experienced an event (in the case of political science, for example).

Ultimately, it will be up to your professor for each class to determine whether or not a source fits his/her idea of primary/secondary, popular/scholarly.