During your job search, a prospective employer may request a list of references prior to or during an interview. This request may take the form of a response to a written job application, a question on a company application or as an addendum to your resume.
The purpose of a reference sheet is to have a list of people who can verify and elaborate on your professional experience for a potential employer. Past employers, professors, and advisors are the best professional references to have. It is important to have a reference sheet because potential employers will often ask for a list of references they can contact.