An access control limits the use of a resource. Only those people, programs or devices that are specifically permitted to use the resource will have access. In addition, an access control will usually limit use to specific types of access; someone can read a file but not change it, for example.
An access control may be physical, for example, a lock on a door; or it may be software or hardware based, for example, in a computer system. It can also be a combination of the two.
In an organization, access management is the sum of all of the practices, policies, procedures, and technical access control mechanisms that are used to appropriately control access to the assets, or a subset of the assets, of the company.