Primary sources tend to stand on their own, while secondary sources are based on other sources, but it is not always easy to discern the difference between the two. The same document, or other piece of evidence, may be a primary source to one researcher and a secondary source to another.
When searching for information on a topic, it is important to understand both primary and secondary sources.
Primary sources are original information. They could be data (e.g. statistics, empirical research, etc.) or written accounts of instances (e.g. letters, diaries, newspaper accounts, etc.) or the actual items themselves (e.g. a historical document, a book, a play, etc.). They should be original and unedited. A primary source requires the learner to interact with the source and extract information.
Secondary sources are edited primary sources. They utilize primary and other secondary sources to evaluate, synthesize and/or merge data. For example, they can introduce someone's interpretation previously published data set, or their opinion/review of a particular play, book or piece of music. Secondary resources do not rely upon the direct observation or participation. They may even rely on other secondary resources in their formation.