Raspberry Pi OS (formerly Raspbian) is a GNU / Linux operating system specially designed and optimized for the Raspberry Pi.
On this page you can download Raspberry Pi OS in its latest version.

Raspberry Pi OS (previously Raspbian)

The latest version of the full Raspberry Pi OS, more suited to the Raspberry Pi 3 and Raspberry Pi 4.
SHA-1: d7a17375c7e84a89b2636d6152a204c6f1a16e03

Raspberry Pi OS Lite (previously Raspbian Lite)

It is possible to download a reduced version of Raspberry Pi OS (especially for Raspberry Pi A + or Raspberry Pi Zero).
SHA-1: 10b2ce7323a65cb12b730f97ff79fd4c8150498f
While your file is downloading, you could make yourself a coffee and drink it while reading our article devoted to the choice of accessories for the Raspberry Pi!