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

Raspbian Stretch

The latest version of the full Raspbian OS, rather adapted to Raspberry Pi 2 and Raspberry Pi 3
SHA-1: 81942d3fcc91799f1f593c076dae893ae13ed820

Raspbian Stretch Lite

You can download a smaller version of Raspbian, rather adapted to Raspberry Pi A+ or Raspberry Pi Zero
SHA-1 : 2083018fb11f070c0e57548b64e2e9a1a3f38d1c
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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