Primary sources are the raw materials of historical research - they are the documents or artifacts closest to the topic of investigation. Often they are created during the time period which is being studied (correspondence, diaries, newspapers, government documents, art) but they can also be produced later by eyewitnesses or participants (memoirs, oral histories). You may find primary sources in their original format (usually in an archive) or reproduced in a variety of ways: books, microfilm, digital, etc.
Note: The definition of a primary source may vary depending upon the discipline or context.
Here are some tutorials, created by librarians and historians, that offer tips on reading and writing history.
Secondary sources offer an analysis or a restatement of primary sources. They often attempt to describe or explain primary sources. Some secondary sources not only analyze primary sources, but also use them to argue a contention or persuade the reader to hold a certain opinion. Secondary sources are not evidence, but rather commentary on and discussion of evidence.
Note: The definition of a secondary source may vary depending upon the discipline or context.
Tertiary sources consist of information which is a distillation and collection of primary and secondary sources.