The terms, periodicals, serials, journals, and magazines are often are used interchangeably. This doesn't mean they are all the same.
Primary sources-- These come first in the publication cycle: objects, original documents or materials containing firsthand information about a topic. When searching Library databases for these materials use the words "sources" or "documents" in combination with your topic. (i.e. slavery and sources).
Secondary sources-- tend to come second in the publication cycle and are about an event or primary source. Secondary sources describe, interpret, analyze, evaluate, explain, comment on, or develop theories related to the primary source. They are written after the fact, with hindsight.
Tertiary sources-- tend to come last in the publication cycle. Generally called "reference." These works list, compile, digest or index primary or secondary sources and can be a good place to look up facts or get a general overview of a subject.