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 Raspberry Pi OS, it is copied onto an SD card using an application like the balenaEtcher or the preferred method, a cross-platform desktop application called the Raspberry Pi Imager, a free download from the Raspberry Pi OS home page. You basically select an image file, select a drive to copy to, then push a button. Done. Then the first time you boot with the SD card you go through an interactive configuration process.
Many Debian-based Linux distributions can be used on a Raspberry Pi. Most of the differences between these are solely in the desktop environment, not the Linux kernel, so if your Pi is being used as a server, or command line only, there's little benefit in an alternative to the "standard". In other words, what are often called "operating systems" are actually all using the Linux operating system and only differing in their desktop environment (window manager).
The default Raspberry Pi OS (including all historical versions of Raspbian) has been 32 bit but in late 2020 the beta version of the 64 bit OS became available.
To determine which is running on a Pi type:
% uname -mIf it responds with "aarch64" then it is 64 bit. If it responds with "armv7l" then it is 32 bit.
The latter is a link to the directory containing RPi OS image files (i.e., not pictures but single-file copies of operating systems), the latest as of this writing is 2020-08-24.
If you never need the UI and plan to run command line only, type this:
sudo apt remove --purge x11-common sudo apt autoremove sudo apt update
This removes all x11 packages, then the 'autoremove' does the cleanup of packages that are no longer needed. Then updates what's left of your operating system.
This will empty up some disk space and speed up operating system updates as there's less on your Pi. As a measure of this, here's the stats on a clean new 64 bit Raspberry Pi OS, before and after removing the UI:
Filesystem Size Used Avail Use% Mounted on /dev/root 7.0G 3.8G 2.9G 58% / /dev/mmcblk0p1 253M 31M 222M 13% /bootAfter:
Filesystem Size Used Avail Use% Mounted on /dev/root 7.0G 3.3G 3.5G 49% / /dev/mmcblk0p1 253M 31M 222M 13% /boot
So this cleared about half a gigabyte of space.