Individual Faculty: You can contact a faculty member to ask about their research and how it aligns with your interests. This is especially important if you are a Ph.D. applicant seeking a potential advisor. However, faculty members may be too busy to respond to inquiries before you submit your application.  
Program Director or Graduate Academic Advisor: These individuals are the experts on a program's structure and requirements. They are the best contacts for questions about coursework, curriculum, and the academic advising process. 
International Student Office: This is the best place to go for any questions about visas, immigration policies, financial documentation for your I-20 form, and work authorizations like OPT and CPT. This office is the final authority on these matters, not the academic department or admissions office.