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.
Primary sources are original information. They could be data (statistics, empirical research, etc) or written accounts of instances (letters, diarys, newspaper accounts, etc) or the actual items themselves (a historical document, a book, a play, . . ). 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 take primary and other secondary sources to evaluate, sythisize and merge the data. They involve someone's opinion of the data, interpretation of the incident or review of a play, book or pieces of music. Secondary resources do not rely unpon the direct observation or participation. They may rely on other secondary resources in their formation.