Audit commitee and internal audit department!

what is the difference in internal audit dept and audit commitee ? if the company doesnt have both , then isit necesary that company will have an INTERNAL AUDITOR ? or if the company have internal audit department , does the INTERNAL AUDITOR would be a part of that INTERNAL AUDITOR DEPARTMENT ? and if the company have internal auditor plus an internal audit department and its establishing the audit committee as well (comprising of 100% NEDs ) , would the audit commitee just supervise the activities of internal audit department ? or what ? plz kindly resolve my confusion ! Thankyou .