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

Raspberry Pi OS (previously Raspbian)

The latest version of the full Raspbian OS, rather adapted to Raspberry Pi 3 and Raspberry Pi 4
SHA-1: 001d385a5c2b611da452b058aa5c36006f2e4392

Raspberry Pi OS Lite (previously Raspbian Lite)

You can download a smaller version of Raspbian, rather adapted to Raspberry Pi A+ or Raspberry Pi Zero
SHA-1 : a3d77244e307d7c1f7052d6ebed9ac472234e5aa
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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