Primary sources are original materials created at the time of the occurrence in question. Some examples of primary sources include the art of the period, portraits, museum collections, newspapers, films, photographs, catalogs, and advertisements.
Secondary sources are based on primary resources, but occur after the occurrence in question, and provide comentary or an interpretation of the occurrence.Some examples of secondary sources are monographs (books on a single subject) and journal articles.
Tertiary sources summarize information based on secondary and primary resources.Tertiary resources are a good places to look for dates and verify terms and/or spelling. They often provide suggested reading lists. Some examples of tertiary resources are encyclopedias, dictionaries, and handbooks.