Typically, historical research can be separated into two categories: Primary and Secondary.
A Primary Source is Defined As: Materials produced by people or groups directly involved in the event or topic under consideration, either as participants or as witnesses. These sources provide the evidence on which historians rely in order to describe and interpret the past. Some primary sources are written documents, such as letters; diaries; newspaper and magazine articles; speeches; autobiographies; treatises; census data; and marriage, birth, and death registers. In addition, historians often examine primary sources that are not written, like works of art, films, recordings, items of clothing, household objects, tools, and archaeological remains.
A Secondary Source is Defined As: A resource that discusses a prior event from an academic or research or layperson perspective. The key here is that the resource is from some time after the event. An example of this would be a book about Jack the Ripper or an article in a journal publication discussing the significance of Jack the Ripper. A book will NOT typically be a primary source unless it is a compilation of letters or diary entries or documents.
Primary vs. Secondary Source: Often times determining whether something is primary or secondary may depend on the question that is being asked. For instance, if a researcher is researching the issues around the Jack the Ripper case and reads a book on that topic published in 1900 that may be determined to be secondary but if the question is then about opinions of the Jack the Ripper case than that same book can then become primary.