Linux

Linux is a free and open source, monolithic, Unix-like, operating system kernel originally developed by Linus Torvalds in 1991. By extension, the term also refers to full operating systems that combine the Linux kernel with a set of software in order to provide a complete working system. These so-called Linux distributions usually bundle GNU tools, which led to the combination also often being called GNU/Linux.

Operating systems such as Android and Chrome OS are also based on the Linux kernel; however, they are not usually considered Linux distributions due to the large differences in their architecture as compared to a conventional GNU/Linux system.

Distributions
Linux distributions are operating systems that bundle the Linux kernel together with other common software, including an implementation of, system daemons and user applications. While most distributions offer the same basic Unix tools, they often differ in their overall focus and purpose, which impacts the choice of bundled application software or settings. Unlike most other commercial operating systems, Linux distributions in general are also highly modular and the used package management solution makes for another key difference across various distributions.

Overall, most Linux distributions can be organized into several groups:
 * Debian and derivatives
 * Ubuntu and derivatives
 * Fedora and derivatives
 * Mandriva Linux and derivatives
 * openSUSE and derivatives
 * Arch Linux and derivatives
 * Gentoo and derivatives
 * Slackware and derivatives

Debian and derivatives

 * Debian
 * Linux Mint (LMDE)
 * antiX
 * Astra Linux
 * AV Linux
 * BunsenLabs
 * Corel Linux (Discontinued)
 * CutefishOS
 * Damn Small Linux
 * Deepin
 * Devuan GNU+Linux
 * Endless OS (Debian Stable)
 * Kali Linux (Debian Testing)
 * KNOPPIX
 * Morphix
 * MX Linux
 * OSMC
 * Pardus
 * Parrot Security (Debian Testing)
 * PureOS
 * Q4OS (Debian Stable)
 * Raspberry Pi OS
 * Red Flag Linux
 * Slax
 * SparkyLinux
 * SteamOS (Debian Stable)
 * Tails (Debian Stable)
 * Ubuntu
 * UOS
 * AmogOS

Ubuntu and derivatives

 * Ubuntu
 * Linux Mint
 * Bodhi Linux
 * Elementary OS
 * Feren OS
 * Freespire
 * JingOS
 * KDE neon
 * Linspire
 * Linuxfx
 * Linux Lite
 * MIKO GNYO/Linux
 * Mythbuntu
 * pearOS
 * Sxolinux
 * Peppermint Linux
 * Pop!_OS
 * Robolinux
 * Trisquel
 * Zorin OS
 * Ubuntu Touch

Fedora, Red Hat and derivatives

 * Red Hat Linux
 * Fedora
 * Red Hat Enterprise Linux
 * CentOS
 * AlmaLinux
 * Asianux
 * Baruwa Enterprise Edition
 * Berry Linux
 * BlueOnyx
 * ClearOS
 * Endian Firewall
 * EulerOS
 * EuroLinux
 * Fermi Linux LTS
 * FreePBX
 * Hanthana Linux
 * NethServer
 * Network Security Toolkit
 * Miracle Linux
 * OLPC OS
 * Openwall GNU/*/Linux
 * OpenAnolis
 * Oracle Linux
 * Qubes OS
 * Rocks Cluster Distribution
 * Rocky Linux
 * SME Server
 * Springdale Linux
 * Ultramarine Linux

Arch Linux and derivatives

 * Arch Linux
 * Arch Linux GUI
 * Anarchy
 * Antergos
 * ArchBang
 * Archcraft
 * Archlabs Linux
 * Archman Linux
 * ArcoLinux
 * Artix Linux
 * BlackArch Linux
 * Chakra
 * Ctlos Linux
 * EndeavourOS
 * FWUL Linux
 * Garuda Linux
 * Hyperbola
 * Mabox Linux
 * Manjaro
 * Obarun
 * Parabola
 * RebornOS
 * SteamOS 3.0
 * Velt/OS

Gentoo and derivatives

 * Gentoo
 * Calculate Linux
 * Chrome OS
 * Chromium OS
 * CloverOS
 * CosmicMod
 * exGENT
 * Exherbo
 * Flatcar Linux
 * Funtoo
 * Gentoo Studio
 * Gentoox (Discontinued)
 * Kogaion (Discontinued)
 * MocaccinoOS
 * PapugLinux (Discontinued)
 * Pentoo
 * PixieLive (Discontinued)
 * Porteus Kiosk
 * Redcore Linux
 * RogentOS (Discontinued)
 * Sabayon
 * Securix (Discontinued)
 * Tizen
 * Toorox (Discontinued)

Mandriva and derivatives

 * Mandriva Linux
 * ALT Linux
 * Mageia
 * OpenMandriva Lx
 * PCLinuxOS
 * ROSA Linux

Slackware and derivatives

 * Slackware
 * Absolute Linux
 * Parted Magic
 * Porteus
 * Salix OS
 * Slackel
 * Slax (until 9.0, now avaliable as a separate 64-Bit version)
 * VectorLinux
 * Zenwalk Linux

Others

 * 4MLinux
 * Adélie Linux
 * Alpine Linux
 * Android
 * AOSC OS (Anthon)
 * Caldera OpenLinux (Discontinued)
 * Checkra1n Linux
 * Clear Linux
 * CBL-Mariner
 * CRUX
 * EasyOS
 * Gecko
 * GNOME OS
 * GoboLinux
 * Guix System
 * KaiOS
 * KISS Linux
 * Kylin OS
 * LibreELEC
 * MCC Interim Linux
 * NixOS
 * NuTyX
 * OpenELEC
 * openSUSE
 * Photon OS
 * Pisi Linux
 * Red Star OS
 * SliTaz
 * Softlanding Linux System
 * Solus
 * T2 SDE
 * TAMU Linux
 * Tiny Core Linux
 * Venom Linux
 * Void Linux
 * Yggdrasil Linux
 * Lakka TV

GUIs
Here provides a list of GUIs that can be used on Linux.

Desktop environments
Various desktop environments offer unique appearances, icon themes, different ability of customization, and many more. A user can switch from one to another by downloading them from the default package manager provided.
 * Budgie
 * COSMIC
 * Cinnamon
 * Cutefish DE
 * Deepin DE
 * Enlightenment
 * GNOME
 * KDE Plasma
 * Lumina
 * LXDE
 * LXQt
 * MATE
 * Moksha
 * Pantheon
 * pearOS (Desktop Environment)
 * Sugar
 * theDesk
 * theShell
 * Trinity DE
 * UKUI
 * Unity
 * Xfce

Standalone window managers
Unlike desktop environments, standalone window managers are only software that controls the appearance and position of windows within a windowing system.
 * awesome
 * BSPwm
 * dwm
 * Fluxbox
 * FLWM
 * FVWM
 * Herbstluftwm
 * i3
 * IceWM
 * JWM
 * LeftWM
 * Notion
 * Openbox
 * Qtile
 * Window Maker
 * Xmonad