Permissions are settings assigned to each file and folder that determine what type of access users can have to the file or folder. For example, you can determine whether other users can read and edit a file that belongs to you, or only have access to read it but not make changes to it.
Each file belongs to a particular user, and is associated with a group that the owner belongs to. The super user "root" has the ability to access any file on the system.
You can set permissions for three categories of users:
The user that created the file or folder.
A group of users to which the owner belongs.
All other users not already included.
For each category of user, different permissions can be set. These behave differently for files and folders, as follows:
Files can be opened
Directory contents can be displayed
Files can be edited or deleted
Directory contents can be modified
Executable files can be run as a program
Directories can be entered
For more on changing the permissions for a file or folder, see Section 6.6.16 ― Changing Permissions.