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Access Control List
The Access Control List (ACL) is exactly as the title implies, a list a permissions users have for computer objects, including folders and specific files.
ACL explicitly says which users or usergroups can access the folder/file and whether they can perform any operations on it, including basic operations like read, write and deleting.
As an example, the ACL will define whether a certain folder is read-only or read/write, meaning either the user/usergroup can only open the file without editing or open it and have full access to editing/writing to it.
There are ACL in all operating systems, as they are the easiest way to manage user permissions, especially in larger offices.
The lists are normally hidden and run by administrators but can be openly modified sometimes in Unix and Mac OS X.