Primary sources are original records, usually created during the time period you are researching, and not distilled or evaluated by someone else. Examples might include newspapers, letters, or diaries. Primary sources may also be created at a later date by a participant or observer of the time period through memoirs, autobiographies, or oral histories.
Whether a document is "primary" depends on the research topic. For example, if your research topic was Health Study X, a newspaper article about Health Study X would not be a primary source. Instead, the primary source would be the original research--the study itself. On the other hand, if your topic was media reactions to Health Study X, the newspaper article would be a primary source.
The two other types of sources are "secondary" and "tertiary." In general, a secondary source evaluates or interprets a primary source. Secondary sources themselves are not evidence; they analyze, critique, or comment on evidence. A tertiary source is a collection of primary and secondary sources, such as an almanac. For further explanation and examples of different types of sources (primary, secondary, tertiary), see this site from the University of Maryland .
Because sources' categorization as primary, secondary, or tertiary is topic-dependent, it is impossible to conduct a Searchworks search of just primary sources. However, you can do an advanced search in Searchworks to help narrow it down. For example, if your topic is Martin Luther King, Jr., you could search King's name, plus "letters" or "diaries." Or you might restrict the date range to the years before his death (to exclude historical articles written about King).
Here is a website with links to primary sources at Stanford, including those at Hoover Archives, Green Library Special Collections, the Canter Arts Center, the Music Library Archives, the Branner Earth Sciences Library, and The Art and Architecture Library.
The library also has access to databases that include primary resources. Check Databases A - Z for databases such as "North American Immigrant Letters, Diaries and Oral Histories" and websites such as American Memory from the Library of Congress.