Access control is way of limiting access to a building or to physical or virtual resources. Access control is a process by which users are granted access and certain privileges to a building and resources or information.
In access control systems, users must present credentials before they can be granted access. In physical systems, these credentials may come in many forms, but credentials that can't be transferred provide the most security.
Physical access control is a matter of who, where, and when. An access control system determines who is allowed to enter or exit, where they are allowed to exit or enter, and when they are allowed to enter or exit. Historically, this was partially accomplished through keys and locks. When a door is locked, only someone with a key can enter through the door, Mechanical locks and keys do not allow restriction of the key holder to specific times or dates. Mechanical locks and keys do not provide records of the key used on any specific door, when a mechanical key is lost or the key holder is no longer authorized to use the protected area, the locks must be re-keyed.