Commentary is defined by The American Century Dictionary as "1) [a] descriptive account of an event or performance as it happens or 2) explanatory notes". Although when I use the word commentary, I mean written comments about the paper by a teacher, tutor, or Writing Fellow to help create a better writer (and hopefully better writing as well); both of the definitions presented in the dictionary help to explain commentary. Writing is an on-going process, usually stopped only by a deadline. Commentary, especially from a Fellow or tutor, is a "descriptive account" of where the reader is in the writing "as it happens". Commentary can also be explanatory notes; commentary explains where the reader (i.e. tutor, Fellow, or teacher) became confused or required more proof to convince him or her.
Commentary is more that scribbled notes in red pen or a note at the end of a paper written by a teacher to prove he or she read the whole paper. Commentary is a dialogue with the reader on paper. There are two types of commentary (directive and facilitative) as well as many ways to approach writing commentary. With time and practice your own style will develop; however, this website will give explanations about commentary, including: definition and example of end comments, examples of commentary, reflections on commentary, my personal struggles with commentary, dos and don'ts of commentary, and commentary techniques. This site will also refer you to other sources to learn more about commentary and the theory surrounding it.
Writing Fellows are not the only peer tutors that may use and see commentary; tutors in the Writing Center setting see commentary as well. Often tutors must interact with a paper which already has commentary on it. The question "Can tutors write commentary?" also arises.