When drawn on paper, this is a 2D representation of the three dimensions. For example, when you draw a cube on a sheet of paper, this is still 2D, but it is a 3D representation, unlike a drawing of a square. 3D movies are labeled as 3 dimensional, but they are really just a 3D representation of a 2D object.
In July 2010, CentOS overtook Debian to become the most popular Linux distribution for web servers, with almost 30% of all Linux web servers using it. Debian retook the lead in January 2012.
Debian is under continual development. The latest release is Debian 8.7. It is also (currently) known as stable or by its codename Jessie. Each version also corresponds to a set of named software repositories (at least one per CPU architecture). At any given time, there is one stable release of Debian, which has the support of the Debian security team. When a new stable version is released, the security team will usually cover the previous version for a year or so, while they also cover the new/current version.