Primary, Secondary and Tertiary Sources
Sources fall into three main categories, primary, secondary and tertiary.
Primary sources give firsthand accounts or direct evidence regarding the event. They are writings contemporary to what is being researched.
Secondary sources discuss information presented elsewhere. It is created later, after the event, by someone who did not participate or experience the event. Most scholarly articles and books are secondary sources.
Tertiary sources consolidate and summarize primary and secondary sources. For example, encyclopedias and factbooks are considered tertiary (although some may be secondary).