Raspberry Pi OS

Raspberry Pi OS (formerly Raspbian) is a Debian-based operating system for Raspberry Pi. Since 2015, it has been officially provided by the Raspberry Pi Foundation as the primary operating system for the Raspberry Pi family of compact single-board computers. The first version of Raspbian was created by Mike Thompson and Peter Green as an independent project. The initial build was completed in June 2012. 
Raspberry Pi OS is highly optimized for the Raspberry Pi line of compact single-board computers with ARM CPUs. It runs on every Raspberry Pi except the Pico microcontroller. Raspberry Pi OS uses a modified LXDE as its desktop environment with the Openbox stacking window manager, along with a unique theme. The distribution is shipped with a copy of the algebra program Wolfram Mathematica and a version of Minecraft called "Minecraft: Pi Edition", as well as a lightweight version of the Chromium web browser.
Features.
User Interface.
Raspberry Pi OS looks similar to many common desktops, such as macOS and Microsoft Windows. The menu bar is positioned at the top and contains an application menu and shortcuts to Terminal, Chromium, and File Manager. On the right is a Bluetooth menu, a Wi-Fi menu, volume control, and a digital clock. 
Package Management.
Packages can be installed via APT, the "Recommended Software" app, and by using the "Add/Remove Software" tool, a GUI wrapper for APT.
Microsoft Repository Controversy.
In late January 2021, Raspberry Pi OS' package added a trusted GPG key and entry to APT without user consent.
This addition granted Microsoft the ability to install and run any software during the daily critical update process on all Pi that had done a manual apt upgrade to receive the change.
The change was not pushed as a critical update and, as of yet, the excessive permission has not been abused by Microsoft and would seem unlikely to ever be abused.
The author of the change acknowledged on GitHub that too many rights were granted to Microsoft and also acknowledged delaying the public release of the source code for the change. 
In addition to the permissions, the change also causes Pi running an updated Raspberry Pi OS to contact daily and thereby reveal their IP address as a Raspberry Pi OS user for potential use in tracking or marketing efforts.
On 8 February 2021, the original author made another change that restricted Microsoft's ability to install software to packages beginning with the string "code" but Microsoft can still run code as root so this restriction is trivial to bypass.
As of 8 February 2021, the issue is not resolved and the Raspberry Pi Foundation has locked or deleted many of the related threads on their public forum and their GitHub pages but has acknowledged there is a problem to be resolved and that they are working on it.
Components.
PCManFM is a file browser allowing quick access to all areas of the computer, and was redesigned in the first Raspberry Pi OS Buster release (2019-06-20).
Raspberry Pi OS originally used Epiphany as the web browser, but switched to Chromium with the launch of its redesigned desktop.
Raspberry Pi OS comes with many beginner IDEs, such as Thonny Python IDE, Mu Editor, and Greenfoot. It also ships with educational software like Scratch and Bookshelf.
Reception.
Jesse Smith from DistroWatch reviewed Raspberry Pi OS (then Raspbian) in 2015:
Based on download statistics from the Raspberry Pi Imager, Raspberry Pi OS is by far the most used operating system on the Raspberry Pi, accounting for 71.88% of all OS downloads in the past month, as of 1 February 2021.
Versions.
Raspberry Pi OS has 3 installation options:
Raspberry Pi OS Lite is the smallest version and doesn't include a Desktop Environment.
Raspberry Pi OS includes the Pixel Desktop Environment. 
Raspberry Pi OS Full also comes pre-installed with additional productivity software.
All versions are distributed as .img disk image files. These files can then be flashed on to microSD cards where Raspberry Pi OS runs. In March 2020, the Raspberry Pi Foundation also published the Raspberry Pi Imager, a custom disk flasher that allows for the installation of Raspberry Pi OS as well as other operating systems designed for the Raspberry Pi, including RetroPie, Kodi OS, and others.
Version size.
The Raspberry Pi documentation recommends at least a 4GB microSD card for Raspberry Pi OS Lite, an 8GB microSD card for Raspberry Pi OS, and a 16GB microSD card for Raspberry Pi OS Full. The image files themselves are 442MB, 1,175MB, and 2,868MB respectively.