Primary v. Secondary Sources
Historical writing is based on information from a combination of primary and secondary sources. One of the key skills necessary in writing history is learning how and when to use which kind of source.
Primary sources provide the raw data that you use to create your historical account. These are mostly written documents produced during the period you are studying, but may include artifacts or oral histories from the time, or accounts written by participants well after the fact.
Secondary sources are written after the fact by historians, journalists, and other authors who were not direct witnesses and who have used historical documents or artifacts or oral histories (primary sources) to produce an account of the past. Secondary sources can be divided into two sorts: monographs, written on a narrow topic and produced largely from primary sources, and overviews which focus on a broader topic and are synthesized from other secondary sources. Some folks call these "tertiary sources."
You should use all three kinds of sources.
If you are doing a research paper (in way, ALL history papers are research papers--including the ones where you have to find your own sources and the ones where the teacher has given them all to you), begin your research with tertiary sources, including your textbook and reference books. The best tertiary sources, however, are overviews and surveys, often written by senior historians seeking to sum up and bring together the latest and most important scholarship in their fields of study. Such books may be organized into chronological surveys, like the excellent Oxford History of the United States series or thematic overviews, like The Story of American Feedom, by Eric Foner. Using these sources first will help you get up to speed quickly on your topic and to see it in its larger context. These sources will also help you identify the most important works on your topic (check out the footnotes and bibliographies), the key questions historians have asked, and different schools of thought.
Monographs, both book- and article-length, are the most significant works on your topic and will give you more in-depth information. See the "Evaluating Sources" page of this website for guidance in how to identify the best secondary source on your topic.
Here are some general guidelines about when to use a secondary source and when to use a primary source as you write your paper:
It's important to figure out what new thing you want to add to the discussion of your topic, and then use primary document evidence to show it. Use secondary sources to show everything else.
Use primary documents to find the most significant evidence supporting your thesis and enabling you to say something original about your topic. You turn facts from primary documents into evidence when you analyze and interpret them in a way that supports the thesis. These interpretations should be yours, not those of other historians. Don't use primary sources uncritically as sources of factual evidence. You need to evaluate the reliability of the source and engage in "source criticism." Always seek to use secondary sources to verify facts that do not require interpretation.
It makes sense to quote directly from primary sources when the words themselves are evidence that support a claim you are making, and also to bring history to life.
Use secondary sources (1.) for facts that are well established, undisputed, or don't require interpretation, and:
2. For background information—facts that don't directly pertain to your argument but that the reader needs to know to follow the story and understand the argument.
3. To fill in gaps in the primary document record—if you need a fact to complete your argument but can't find it in a primary source you may now and then use a fact from secondary sources that helps to advance your argument. If you do too much of this, however, your essay will lack originality--especially if it's a "research paper." You may also use quotes from primary sources that appeared in the secondary source (again, this reduces the originality of your paper).
Finally, use secondary sources (4.) when the authors are well-known authorities and you want to enhance the credibility of an assertion by using their words to express that assertion or (5.) as foils for your own argument: as you do your research you will find historians who have studied the same topic and asked the same questions of it. Using them as foils by saying how your research disagrees with or advances their arguments. For more on using other historians as foils, see "Responses" on my Ask Better Questions page.
***You should most often summarize or paraphrase rather than quote directly from secondary sources when using them for the first three reasons; quoting makes more sense when using them for reasons 4 and 5. Do NOT use quotes from secondary sources to save yourself the trouble of composing parts of your own essay.