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: 7b5c6e106358d35655770d6f015dde5f15558a78

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: 1259083f1df10e2ec24cfae69af54baedb45227b
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!