Legacy Unix recognizes only a limited set of access control rules. Basically, read/write/execute for owner/group/guest users.
SeLinux was designed to provided a finer-grade set of access rules, similar to the level of refinement you get from systems like Windows NT, but with better attention to locking things down by default. For example, you might permit user "apache" to be able to modify the apache config files, but allow root to only view them. In a non-SeLinux system, root can do anything to anybody.
selinux comes available with a lot of current-generation Linux distros. I think that the Red Hat Enterprise releases have it turned on by default, since servers are especially vulnerable to exploitation.
It would take too much time to explain in detail, but here's a good starting point:
http://en.wikipedia.org/wiki/SELinux In RHEL, the specialized security users, groups, and rules are mostly provided by Red Hat as part of the package installs. RHEL 5 is suppsed to have come up with a management console app that makes it easier for you to set up and control your own rules.
Two useful things to know where you work with selinux:
ls -Z will show the filesystem object's selinux attributes. You can stack it with traditional ls options (for example: ls -lZh /var/www).
The program that allows you to set selinux attributes (a la chmod) is called "chcon". Which I can never remember.

Only use if if you are very brave, however, since you can potentially lock yourself out of critical system controls. Most of the chcon stuff is done by selinux-aware package installers, so you won't have to.