The Unix operating system (and likewise, Linux) differs from other computing environments in that it is not only amultitasking system but it is also a multi-user system as well.

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In order to make this practical, a method had to be devised to protect the users from each other. After all, you could not allow the actions of one user to crash the computer, nor could you allow one user to interfere with the files belonging to another user.

This lesson will cover the following commands:

• chmod – modify file access rights
• su – temporarily become the superuser
• chown – change file ownership
• chgrp – change a file’s group ownership

Full article here:
Understanding Linux File Permissions (Project: Fenix)