SELinux is a combination of kernel changes and administration utilities that extends the Linux kernel's security model to implement more specific capabilities and restraints. Notably, SELinux can constrain a running process's privileges based not just on the process's owner, but on its identity. While there are some differences in design principles, SELinux is similar to AppArmor (which is newer, and more commonly used on Ubuntu).
SELinux is a security enhancement to Linux which allows users and administrators more control over access control.
Access can be constrained on such variables as which users and applications can access which resources. These resources may take the form of files. Standard Linux access controls, such as file modes (-rwxr-xr-x) are modifiable by the user and the applications which the user runs. Conversely, SELinux access controls are determined by a policy loaded on the system which may not be changed by careless users or misbehaving applications.
Source:SELinux Project
