Primary sources are the raw materials of scholarship - original sources of information that have not yet been filtered through analysis, examination or interpretation. Primary sources are often contemporary to the events and individuals being researched. Typically, when we speak of primary sources in literature we mean either the literary work under study (Milton's Paradise Lost) or personal information produced by the author himself (letters, diaries, manuscripts, and archival papers).
Scholarly and critical editions will typically present and discuss variant versions of a text; provide historical, cultural and linguistic context; and delineate the history of scholarly approaches to that text. Most scholarly editions will have an editor (or editors) and will be published by a university press. These editions are especially useful for approaching works with complicated textual histories.