Peer review is the process by which articles or other works are critiqued before they are published. Authors send articles to an editor, who decides whether the work should be forwarded to reviewers for the journal. The most stringent form is anonymous or blind review, where neither the author nor the reviewers know whose work is being examined by whom. This helps reduce bias.
Reviewers are usually well-published researchers and experts themselves. The articles are sent back to the editor with remarks and recommendations, usually 1) publish as is (rare), 2) publish if edited or changed in specific ways, or 3) don't publish. Editors will usually go with the recommendation of the majority of the reviewers. If revision is recommended, the reviewers' comments may be returned with the draft. Authors who have been asked for revisions normally make the recommended changes and resubmit the article to the editor.
The process is intended to improve the content of studies published-- more eyes on a project, and one's reputation on the line with peers, tends to improve the quality of what's published. There are cases where it hasn't worked, and critics of the cycle, but it is the best system that has been developed to this point.
What is considered a primary source varies somewhat by discipline. In any case, think of a primary source as first-hand knowledge, eyewitness accounts,or reports about (X topic).
In the arts, a primary source may be a piece of art, such as a painting or sculpture, a musical score, a poem, a book or chapter, or an essay--whatever is created by the artist, writer, photographer, etc.
In the sciences, a primary source is the first report of research; it may be published as a journal article, or sometimes as a research report or conference presentation.
In some of the social sciences, such as anthropology, ethnography, psychology, sociology or social work, a primary source may be the first report of a piece of research, especially of empirical studies, or it may be something closer to primary sources in history, since some areas of these fields depend on direct observation, data, personal narratives or commentary, as from interviews or case studies.
For history, a primary source is a letter, a diary, speech, lecture, piece of legislation, document or manuscript-- an original source which forms the basis, with other sources, of secondary work, such as a study of life in eighteenth century Ireland. A narrative is usually a personal account, by a single individual, of a period of time or an event.
A secondary source is based on a primary source or other sources. It includes analysis, criticism, or other intellectual input. Secondary sources can include books, book chapters, articles, especially literature reviews, and some book reviews.
A tertiary source is commonly a resource or tool that helps people find primary or secondary sources. Tertiary sources include most bibliographies, databases and indexes, and library catalogs.
What's gray literature? Gray literature is information that is usually not published for the general population, but available in limited distribution, typically to persons inside a company, a discipline, an industry, or a government sector. Examples include white papers, preliminary reports, brochures, handouts, working papers, notes, and so on. Some gray literature, such as in as scientific or engineering areas, is fairly well indexed and easy to find, and can be useful in specific circumstances.