As a field, security encompasses a wide variety of techniques and principles, and includes such areas as physical security, personnel security, computer security, legal issues, and cryptography. Our concern is considerably more focussed. We are only concerned with three security objectives in our discussions; privacy, integrity, and availability.
Notice that when we talk about security objectives, we talk in terms of people. That is our major concern; assuring that people can do what they are supposed to do and can not do what they are not supposed to do. In Unix(tm), people roughly correspond to users, and act through processes. Processes interpret user keystrokes, and act for the user in manipulating processes and files.