SELinux is a fairly new development in the realm of secure Linux systems. It was developed by the National Security Agency (NSA) in the United States, and presumably fits into its mission of securing U.S. computers and communications. But it's curious that a government agency whose raison d'être includes making it possible to break into people's computers and wiretap their communication would develop a Linux system that is supposed to be more secure against these kinds of attacks. See the book SELinux (O'Reilly) for an in-depth guide.
SELinux contains a changed Linux kernel that includes mandatory access controls, as well as a number of utilities for controlling the new kernel features. With SELinux, user programs (and daemons) only get just as much access to resources as they need. A security hole such as a buffer overflow in a web server can therefore not compromise the whole computer anymore. In SELinux, there is no root user that has access to everything.
It would be beyond the scope of this book to describe the installation and day-to-day operation of SELinux, but if you are interested in hardened Linux systems, you should have a look at http://www.nsa.gov/selinux. Information about how to install an SELinux kernel on a number of distributions can be found on http://selinux.sf.net.