Linux

Linux (also known as GNU/Linux) is an open source monolithic Unix-like POSIX-compliant kernel created by Linus Torvalds. Unlike some complete operating systems, Linux is a base for OS developers to create an OS with the software of their own while using a pre-made widely-supported kernel. It would become a full fledged operating system 2 years later by Richard Stallman when GNU and Linux were both combined into one.

Everything is a file
In Linux, even things like hardware are represented as files that can be written to. For example, the file for the first hard drive is /dev/sda. This means that if you want to write bytes directly to the drive you can use an app that lays bytes (like dd) to write directly to that file. This can be done by using the app named echo to output something and piping the output to dd which you specify the hard disk's file too.

There are also more liberal concepts of devices in /dev. /dev/null, when written to, discards all data sent to it. If you write to /dev/null and then open it it will still be empty.

There is also another directory, called /proc, that manages other devices. The directory /proc/acpi contains all sorts of subtle devices like the power button and battery.

Freedom
Unlike other operating systems, Linux lets you do what you want out of the box with no restrictions provided. From changing the appearance, to downloading programs that let you edit the operating system, and to view the source code for the purpose of building a custom operating system from scratch.

"Keep it simple, stupid"
Distributions like Arch Linux, Linux Mint, etc. follow a policy called the "Keep it simple, stupid" which means that if possible, complexity should be avoided and everything should be kept as simple as it can be for the user to understand it with ease. This is often seen with distributions that aim to provide Windows and macOS users an alternative with no learning curve needed.

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
 * Cinnamon
 * Cutefish DE
 * Deepin DE
 * Enlightenment
 * GNOME
 * KDE Plasma
 * Lumina
 * LXDE
 * LXQt
 * MATE
 * Moksha
 * Pantheon
 * pearOS (Desktop Environment)
 * Sugar
 * theDesk
 * theShell
 * Trinity DE
 * UKUI
 * Xfce

Distributions
Linux comes in several distributions with their own package managers and other implementations of things. Many Linux distributions include the GNU suite of essential software, however, a notable few (such as Google's Android) omit GNU and instead opt for their own custom suite of software.

Debian and derivatives

 * Debian
 * Linux Mint (LMDE)
 * antiX
 * AV Linux
 * BunsenLabs
 * Corel Linux (Discontinued)
 * CutefishOS
 * Damn Small Linux
 * Deepin
 * Devuan GNU+Linux
 * Endless OS (Debian Stable)
 * Kali Linux (Debian Testing)
 * KNOPPIX
 * MX Linux
 * 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

Ubuntu and derivatives

 * Ubuntu
 * Kubuntu
 * Linux Mint
 * Lubuntu
 * Ubuntu Kylin
 * Ubuntu MATE
 * Xubuntu
 * Bodhi Linux
 * Edubuntu
 * Elementary OS
 * Feren OS
 * Freespire
 * JingOS
 * KDE neon
 * Linspire
 * Linuxfx
 * Linux Lite
 * MIKO GNYO/Linux
 * Mythbuntu
 * pearOS
 * Ubuntu Studio
 * Peppermint Linux
 * Pop!_OS
 * Robolinux
 * Trisquel
 * Ubuntu Budgie
 * Ubuntu Touch
 * Zorin OS

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
 * Oracle Linux
 * Qubes OS
 * Rocks Cluster Distribution
 * Rocky Linux
 * SME Server
 * Springdale Linux

Arch Linux and derivatives

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

Gentoo and derivatives

 * Gentoo
 * Chrome OS
 * Chromium OS
 * CloverOS
 * CloudReady
 * 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)
 * 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)
 * VectorLinux
 * Zenwalk Linux

Others

 * Adélie Linux
 * Alpine Linux
 * Android
 * AmogOS
 * AOSC OS (Anthon)
 * Caldera OpenLinux (Discontinued)
 * Checkra1n Linux
 * Clear Linux
 * CBL-Mariner
 * CRUX
 * EasyOS
 * Gecko
 * GNOME OS
 * GoboLinux
 * Guix System
 * KaOS
 * KISS Linux
 * Kylin OS
 * NixOS
 * NuTyX
 * openSUSE
 * Photon OS
 * Pisi Linux
 * Puppy Linux
 * Red Star OS
 * TAMU Linux
 * Tiny Core Linux
 * Venom Linux
 * Void Linux
 * Yggdrasil Linux

Trivia

 * Linus Torvalds originally meant to call Linux "Freax". One of Linus' coworkers didn't like that name and uploaded the software to the FTP server as "Linux".
 * Linus was bitten by a penguin one time, and that is the reason that the logo of Linux is a penguin.
 * Linux runs most of the servers, and is supported by it's community and big companies such as Microsoft, IBM, and more.