A secondary source of information is one that was created later by someone who did not experience first-hand or participate in the events or conditions you’re researching. They discuss, interpret, comment upon, analyze, evaluate, summarize, and process primary sources. For the purposes of a historical research project, secondary sources are generally scholarly books and articles. Be sure to consult the bibliographies in secondary sources to find possible avenues to primary sources.
Reference sources such as encyclopedias are frequently based upon secondary sources and are, therefore, often considered tertiary sources. While you might not want to use them as one of your sources for your final research, encyclopedias can provide context or background information and their bibliographies can lead you to additional primary and secondary sources.