Based on UNIX systems security model, the Linux security model is as rigid as the UNIX security model--and sometimes even more robust. In Linux, every file is owned by a user and a group user. Those users that are not the user owner and don't belong to the group owning the file are in a third category or users. For each of the three categories of users, the following permissions can be granted or denied: Read Write Execute You have already used the long option to list files using the ls -l command; this command also displays file permissions for these three user categories. They are indicated by the nine characters that follow the first character (the file type indicator at the beginning of the file properties line). Permissions are always presented in the same order: read, write, execute for the user, the group, and the others. With Windows, there is always uncertainty. Files permissions are one of the most confusing issues for Windows defectors. Home-user Windows systems do not have any concept of file ownership…which can be a good or bad thing. Remember that Linux is a UNIX system at heart, and UNIX supports multiple users.
Even on home computers, permissions allow you to block sensitive files from being edited or read by unauthorized people. Consider your old Windows box. Maybe you spent hours or days getting your preferences and settings just right, only to find that the next time you sit down at the computer, someone has changed the colors to purple and neon green and changed the system time to GMT when you live in EST. Then imagine you spend all weekend finalize your big report for your supervisor…only to find out that your child has just discovered the joy of "delete,” and now it is lost forever.