Primary sources are where original research is reported. That information, over time, makes its way into secondary sources, while the tertiary sources are the resources used to search primary/secondary sources.
Knowing which one(s) you need will help you determine what to use to locate them. For research articles, you'd go to databases like Web of Science, and even Google Scholar. But for those secondary sources, you may need to search the databases (for review articles), the Library catalog (for books), or consult one of our research guides (for a list of encyclopedias). But you won't be able to find research articles in the Library catalog.
Adapted from MMW / MCWP guides