Interesting article on Linux permissions. It explains how to recognise which permission system might be in use (SELinux, ACLs), the meaning of the special bits (setuid, setgid, sticky), how umask works, and more.
The basic Linux permission model — owner, group, others with read/write/execute — is just the beginning. Once you start dealing with shared directories, web servers, or multi-user systems, you will need to understand the more advanced features too.
Read the full article on Opensource.com
Enjoy!