There are a number of different definitions for primary literature. These can vary according to the academic discipline (ie. science or humanities). In brief:
These are original materials which have not been filtered through interpretation, condensation, or, often, even evaluation by a second party; for example journal articles, monographs, reports, patents, theses, diaries, letters, photographs, poems.
Primary sources are original materials on which other research is based
They are usually the first formal appearance of results in the print or electronic literature (for example, the first publication of the results of scientific investigations is a primary source.)
They present information in its original form, neither interpreted nor condensed nor evaluated by other writers.
They are from the time period (for example, something written close to when what it is recording happened is likely to be a primary source.)
Primary sources present original thinking, report on discoveries, or share new information.
scientific journal articles reporting experimental research results
proceedings of Meetings, Conferences and Symposia.
dissertations or theses (may also be secondary)
sets of data, such as census statistics
works of literature (such as poems and fiction)
interviews, surveys and fieldwork
letters and correspondence
newspaper articles (may also be secondary)
photographs and works of art
original documents (such as birth certificate or trial transcripts)
Internet communications on email, listservs, and newsgroups