What is the difference between primary and secondary sources and when should they be used?

Primary sources are written by those who experienced the topic in question first hand. They come in a variety of different formats including but not limited to journal entries, letters, recorded interviews, newspaper articles, maps, photographs and so on.

Secondary sources include writing that has been done about an event, often included is an analysis of primary sources. These include journal articles, books, lectures, and so on.

When writing an academic essay, it is crucial to understand what has already been written about the subject whether you agree or disagree with what has been said. As you begin writing a historical research paper you must include a Historiography, or a synopsis of the other work done on your topic (secondary sources), highlighting the ideas presented. 

Throughout your essay when highlighting a certain area or trying to make a specific point about an event you can use primary sources to assist your argument.