Raspberry Pi OS is the offical operating system of the Raspberry Pi (previously known as Raspbian). Though not the only Operarting Systems the Raspberry Pi can use, it is the one that has the setup and software managed by the Raspberry Pi foundation. Raspberry Pi OS is a Linux OS based on Debian.
As with all Linux distributions there are 1000’s of free Open Source programs that can be downloaded from a Terminal Screen or a Desktop app. This section list the 1000’s of programs that can be downloaded for the current version of Raspberry Pi OS, Buster version 10 and the previouse version, Stretch.