All of the files on a Linux system have permissions that allow or prevent others from viewing, modifying or executing. The super user "root" has the ability to access any file on the system. Each file has access restrictions, user restrictions and have an owner/group association.

Every file is secured by the following three sets of permissions, in order of importance:

Inside each of the three sets of permissions are the actual permissons. The permissions, along with the way they apply differently to files and directories, are outlined below:

To view and edit the permissions on files and directories, open the Places->Home Folder and right-click on a file or directory. Then select Properties. The permissions exist under the Permissions tab and allow for the editing of all permission levels, if you are the owner of the file.

To learn more about file permissions in Linux, read the file permissions page in the Ubuntu Wiki.