Debian is a non commercial GNU/Linux distribution that contains more a large choice of software developed and maintained by thousands of developers. Debian is well known for its easiness and its packet manager (APT), using .deb files for installation and updates of the software.
Since the end of the official support of Debian 6 (Squeeze), a project to maintain a LTS (Long Term Support) version was set up.
It is entirely managed by voluntaries and companies who want to have a Debian version that is supported for a maximum of time.
Therefore it is not handled by the regular Debian security team, this may cause that some packets are not necessary updated in terms of security if you use a LTS version.