Q. What's the difference between primary and secondary sources?

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.

Examples include:
Artifacts (e.g. coins, plant specimens, fossils, furniture, tools, clothing, all from the time under study)
Audio recordings (e.g. radio programs, oral histories)
Diaries
Internet communications on email, listservs
Interviews (e.g., oral histories, telephone, e-mail)
Journal articles published in peer-reviewed publications
Letters
Newspaper articles written at the time
Original Documents (i.e. birth certificate, will, marriage license, trial transcript)
Patents
Photographs
Proceedings of Meetings, conferences and symposia
Records of organizations, government agencies (e.g. annual report, treaty, constitution, government document)
Speeches
Survey Research (e.g., market surveys, public opinion polls)
Video recordings (e.g. television programs)
Works of art, architecture, literature, and music (e.g., paintings, sculptures, musical scores, buildings, novels, poems)
Websites

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.

Examples include:
Bibliographies (also considered tertiary)
Biographical works
Commentaries, criticisms
Dictionaries, Encyclopedias (also considered tertiary)
Histories
Journal articles (depending on the disciple can be primary)
Magazine and newspaper articles (this distinction varies by discipline)
Monographs, other than fiction and autobiography
Textbooks (also considered tertiary)
Web site (also considered primary)