"Primary sources are documents, images or artifacts that provide firsthand testimony or direct evidence concerning an historical topic under research investigation. Primary sources are original documents created or experienced contemporaneously with the event being researched. Primary sources enable researchers to get as close as possible to what actually happened during an historical event or time period" ("What are Primary Sources?", UCI Libraries).

Example: interviews of formerly enslaved people

"Secondary sources are works that analyze, assess or interpret an historical event, era, or phenomenon, generally utilizing primary sources to do so. Secondary sources often offer a review or a critique. Secondary sources can include books, journal articles, speeches, reviews, research reports, and more. Generally speaking, secondary sources are written well after the events that are being researched. However, if an individual writes about events that he or she experienced first hand many years after that event occurred, it is still considered a primary source ("Secondary Sources", UCI Libraries).

Example: a book published in 1995 that discusses the experiences of people who were enslaved, which uses interviews, diaries, and statistics to make the claims, etc.

Example: the above example would be primary if you were interested in studying how people in the 1990s wrote about people who were enslaved

"Tertiary sources are sources that identify and locate primary and secondary sources. These can include bibliographies, indexes, abstracts, encyclopedias, and other reference resources; available in multiple formats, i.e. some are online, others only in print.

"It is important to note that these categories, i.e. secondary and tertiary, are not mutually exclusive. A single item may be primary or secondary (or even tertiary) depending on your research topic and the use you make of that item" ("Tertiary Sources", UCI Libraries).

"Use reference books also called reference sources or background sources to get quick specific facts or information or an overview of a subject.

"Some examples of reference sources are: dictionaries, encyclopedias, bibliographies, almanacs, directories, atlases, and handbooks. These can be online or in print" ["What are reference books (or reference sources)?", Simon Fraser University Library].

Reference books can also include a mixture of source types, such as primary sources (text of historical documents, statistics, etc.), secondary sources (analysis of the historical documents, statistics, etc.), and tertiary sources (bibliographies, references, etc).

Scholarly books are published by a university press or other academic publisher.

For History, while historians do write scholarly articles, scholarly books are the gold standard when comes to sharing their research.

Scholarly articles report on the results of a study. They are written by researchers for other researchers and are published in scholarly journals. 

For researchers in the Arts & Humanities, scholarly articles will read more like essays, rather than reports on scientific experiments. For History, a scholar may look at the primary source documents from the time period they are studying to draw conclusions.