Primary Sources are materials that contain direct evidence, first-hand testimony, or an eyewitness account of a topic or event under investigation. They can be published or unpublished items in any format (the original or a surrogate format such as a photocopy, a digital copy, a printed edition, or a microfilm edition), from handwritten letters, to objects, to the built environment.

Secondary Sources are works that analyze and interpret other sources. They use primary sources to solve research problems.

Primary vs. Secondary - The way you engage with a source determines whether it is a primary or secondary source for your project. Book reviews, for example, are typically considered secondary sources.  If the subject of your research is book reviews themselves, however, they would be primary sources for your project.

(Sources:  The Craft of Research by Wayne C. Booth, Gregory G. Colomb, Joseph M. Williams. Chicago: University of Chicago Press, c2008; Introduction to Archival Terminology, NARA.)

Unpublished archival materials are just one type of primary source.  In The Information-Literate Historian, Jenny Presnell describes nine categories of primary sources:

1. Public Records - census records, court records, wills, tax records, etc.
2. Official Records - laws, civil codes, legislative hearings, treaties, etc.
3. Personal Documents (manuscripts) - letters, diaries, oral histories, financial records, etc.
4. Artifacts/Relics - clothing, furniture, tools, music, art, and other items people make and use
5. Organizational Documents (archives) - meeting minutes, financial records, correspondence, etc.
6. Images - photographs, drawings, cartoons, posters, videos, graphics, paintings, etc.
7. Architecture, City Plans, and Maps - buildings, blueprints, plans, models, etc.
8. Media and Other Mass Communication - newspapers, magazines, journals, radio, tv, twitter, facebook, websites, etc.
9. Literary Texts - novels, plays, poems, essays, etc.

(Source:  The Information Literate Historian, 93-95.)

As you conduct your research you may find that some sources will help answer your research question and others will inspire more questions. 

Manuscripts and archives are just one piece of the research puzzle, one kind of "document" to consider when gathering evidence.  As you consider your research question, ask yourself:

• What types of sources might exist?
  Your research question will help you focus your search.  Studying immigrant populations in 21st century New York City will lead you to investigate an entirely different universe of sources than will research on child labor in 19th century textile mills in New England. 
• What kind of sources do you hope to find?
  Different formats fill in the record in unique ways.  Do you want to find personal papers (letters, diaries, documents) or organizational records (correspondence, minutes, publications)?  Contemporary newspaper accounts?  Radical periodicals?  Oral histories?  Photographs or moving images?  Statistics?  Maps?  Government documents?  Scholarly books and articles? Objects?  Printed ephemera?  Works of art?
• Where might the sources have been produced and by whom? 
  Review the holdings of repositories in the geographical area central to your topic. Chances are, a local library, historical society, research library, municipal archive, or university library in the region will hold archival collections and other materials on the subject.  Schools and private businesses might be likely repositories too.
• Could relevant materials be in private collections?
  Catalogs, databases, and repository websites can only help you find materials held in formal institutions. Don't forget that not all sources -- be they manuscripts, printed items, photographs, etc. -- are in libraries.  The more you know about your topic, the easier it will be to discover privately held materials.
• Be creative!  Primary sources are everywhere. 
  When searching for sources, think about how your topic fits in with the world at large.  Considering an era in terms of the political climate, economic picture, popular culture, world events, art, music, literature, science, fashion, media, and the natural environment may spark ideas that lead you to investigate research angles that may not have occurred to you otherwise.

Image:  New York Times