Raspbian is a GNU/Linux operating system specially designed and optimised for Raspberry Pi.
You can download latest Raspbian version on this page.

Raspbian Jessie

The latest version of the full Raspbian OS, rather adapted to Raspberry Pi 2 and Raspberry Pi 3

Raspbian Jessie Lite

You can download a smaller version of Raspbian, rather adapted to Raspberry Pi A+ or Raspberry Pi Zero
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While downloading your file, you could do a coffe and drink it by reading our article on the choice of Raspberry Pi accessories