Raspbian is a Debian based Linux operating system for the Raspberry Pi. This OS has been developed for the RPi and it's features so is the recommended OS to use.
When the Raspberry Pi was the released in 2012 one of the main contenders for the main operating system was Debian Wheezy, a version of Debian which was developed for devices with the ARM processor such as the Raspberry Pi. As Wheezy is a general ARM release it did not take advantage of the Raspberry Pi's hardware where significant speed increases could be made. A small group of developers got together and started optimising the software that is available for Debian Wheezy specifically for the Raspberry Pi and released a new distribution as Raspbian.
Raspbian has a simple desktop environment with the familiar layout of a menu bar at the base of the screen and a programs menu in the bottom left corner. It is a good choice for people who are not use to a Linux operating systems, it also requires less technical knowledge to use than some other operating systems. There is also a wide support community with lots of guides and tutorials available online. See the Web Directory section of RaspberryConnect for many helpful websites.
As Raspbian is a Linux operating system it has good security features, has excellent networking and capabilities and access to 1000's of free programs and utilities called packages that can be installed simply from the terminal. So if you are a casual user and just want Internet and entertainment software or you are interested in programming or electronics there will be software for you only a few key strokes away.
The Raspberry Pi is not a high powered device compared to a desktop computer so some programs will seem a bit sluggish to what you may be use to, for example web browsers are a bit slower with less features but still perfectly usable. With the software optimization the Raspberry Pi can play HD video and music without a problem. If you would like to watch videos which use the Mpeg2 format such as DVD video from home videos you will need to buy a licence first from the Raspberry Pi foundation for a couple of £ due to licencing restrictions.
The Raspberry Pi foundation has used Raspbian as their main supported operating system and have released their own version which has a selection of programs pre-installed for the educational use of the Raspberry Pi such as Scratch, Python and the Raspberry Pi store. This version should be considered the recommended distribution and is available from the Raspberry Pi foundation.
More information about Raspbian including how to download and install it is available at www.raspbian.org
The recommended RaspberryPi Foundation release of Raspbian is available at www.raspberrypi.org/downloads
A list of the 1000's of Raspbian software packages are available on RaspberryConnect in the Raspbian Software Packages section of the Software menu or here
There is also a guide on how to install the packages from the desktop.