This page catalogues operating systems used for personal robots.
"Raspian", a portmanteau of "Raspberry" and "Debian", was the original Linux-based OS for the Raspberry Pi. The current version is still Debian-based, but has been renamed as "Raspberry Pi OS".
To install the OS on a Pi, it is copied onto an SD card using an application like the Balena Etcher, but the preferred method is to an application called the Raspberry Pi Imager, a free download from the Raspberry Pi OS home page.