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: ffb181b3cc483f4142fa3f6f3188968cbb37a723

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: 7a364204e920931701c9859f74a0cf5067ccbbe0
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!