Unix and GNU/Linux systems are known to have a straight forward strategy for file access rights. It categorize users into different groups like, owner, group and the world. Above these groups, there is admin user, who can virtually do anything. Here is an article written by CsarGrnds, published in teknixx , that describes Unix and GNU/Linux file systems in detail.
The Unix operating system (and likewise, Linux) differs from other computing environments in that it is not only a multitasking system but it is also a multi-user system as well.What exactly does this mean? It means that more than one user can be operating the computer at the same time. While your computer will only have one keyboard and monitor, it can still be used by more than one user. For example, if your computer is attached to a network, or the Internet, remote users can log in via telnet or ssh (secure shell) and operate the computer. In fact, remote users can execute X applications and have the graphical output displayed on a remote computer. The X Windows system supports this.