Primary Literature
In philosophy, primary literature is scholarship that primarily attempts to create a new theory or answer a question in a different way. It is sometimes called 'positive philosophy.'
Examples:
Aristotle's Nichomean Ethics
Immanuel Kant's Critique of Pure Reason
Before the 20th century, most published philosophy was primary literature published in books.
Secondary literature
Secondary literature is scholarship whose primary purpose is to comment on someone else's philosophical theory or argument. It is sometimes called 'negative philosophy.'
Examples:
Wood, James L., "Contemplating the Beautiful: The Practical Importance of Theoretical Excellence in Aristotle’s Ethics", Journal of the History of Philosophy, 49(4), Oct 2011, pp. 391-402.
Cohen, Alix A., "Kant’s Concept of Freedom and the Human Sciences", Canadian Journal of Philosophy, 39(1), Mar 2009, pp. 113-135.
Before the end of the 19th century, criticism largely took place in letters, lectures and other unpublished materials. In the late 19th century, journals took over as the primary avenue for published criticism, though the older criticism channels are still used. Today, most criticism is published in journals, or books composed of group of articles.