Categories and information described here are not quite so iron-clad and distinct as the definitions below suggest, but they may offer some general guidance on how to identify and search for these different types of research sources.
Primary sources are items that are directly associated with their producer or user and the time period in which they were created. Examples include diaries, newspaper articles, government documents, clothing, photographs, oral interviews, and news broadcasts. (The Information-Literate Historian, by Jenny L. Presnell).
(1) Library Catalog Searching
When searching in the library catalog, try adding a word for the source type with your topic for example terms such as correspondence diaries or personal narratives to retrieve books that have primary source material.
(2) Consult Reference Books
Many reference books include sections of primary documents related to their topic of coverage. For example, the 5-volume set, The Encyclopedia of the American Revolutionary War: A Political, Social, and Military History, ed. by Gregory Fremont-Barnes & Richard A. Ryerson, includes an entire volume (volume 5) of "documents," about this topic.
(1) Immediacy of Source
Are the documents (newspapers, interviews, etc.) recounting events experienced by the narrator (primary) or reporting the experiences of others (secondary). The account may be primary to the time and narrator (speaker, writer, time period), but not always to the specific event being discussed.
(2) Format of Source
The "primary" nature of the document is intrinsic to the content, not the format, of your source. Whether handwritten, recorded, printed, or digital, you should be concerned about who is writing (or recording), and what is the relationship to the account reproduced.
Secondary works are analyses and interpretations based on primary sources and other sources, which can include other secondary works such as books and journal articles, and tertiary sources such as encyclopedia articles. (The Information-Literate Historian, by Jenny L. Presnell)
(1) Journal Article Indexes & Library Catalog
Scholarly, academic research journal articles (or books) are often the "secondary" literature your professors mean when they direct you to this type of resource. Works that analyze, discuss, or otherwise reference original sources or other scholarship fall into this category. Article indexes or the library catalog are tools to help you locate and explore this content.
Content vs. Format There is nothing that innately prevents a journal or book from presenting you with primary data, or empirical research directly observed or gathered by the author. But often academic scholarship (books & journals) of a historical nature treat and discuss documents, events, and texts from earlier eras, making these works "secondary" research.
A term scholars sometimes use for a reference work intended to help a researcher identify primary and secondary sources. This includes finding tools such as catalogs, indexes, bibliographies, and encyclopedias. (The Elements of Library Research: What Every Student Needs to Know, by Mary W. George).
(1) Handbooks, Manuals, etc.
Quite often, a tertiary source will contain the words "handbook," "companion," "manual," or "guide," in its title. Terms like these will alert you that the book is summarizing and guiding you to other sources. These works are designed to give a "snap-shot" of a scholarly discipilne at a particular point in time. Library catalog subject headings frequently have the phrase "handbooks, manuals, etc.," as keywords you can use along with the rest of your catalog search.
(2) Encyclopedias & Dictionaries
These sources, along with many other books in a typical library's reference collection, are usually tertiary. They offer definitions, summary data and information about the subject, and frequently a list of references for further study. Since students are often assigned to engage with actual primary and secondary scholarship, rather than works about this material, they are usually advised to consult tertiary sources as needed, but to refrain from using them as sources for their own writing. You are to summarize and discuss the scholarship that you have been asked to research, not discuss someone else's!
(1) Just Use Wikipedia! (Well, ... no.)
Articles you encounter on this well-known resource look highly informative and polished. That is, indeed, how they look. The website creators are quick and candid to explain, however, that (among other important rules): (a) Wikipedia articles must not contain original research; (b) Users should be aware that not all articles are of encyclopedic quality from the start: they may contain false or debatable information. If you do not already have some expertise in the topic you are researching, this resource poses special challenges to you that you do not encounter (or, we hope, encounter with less frequency) in books and scholarly journal articles that have been selected and reviewed prior to their addition to our library collection.