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.
Linux is a kernel. An operating system that uses the Linux kernel is known as a Linux distribution ('distro' in short). Some of these are also called GNU/Linux distributions because they mostly use GNU packages. Android is a popular exception, though. The Debian operating system is a GNU/Linux distribution. It is one of the earliest linux distros and is still popular to this day. It has served as a base for many other distros like Ubuntu, Mint, Kali, etc.