Linux

Linux is a kernel that is used as the base of Ubuntu, Debian, Fedora, Android, and Remix OS.

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.

Linux OSes
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
 * Devuan GNU+Linux
 * Linux Mint (LMDE)
 * antiX
 * SparkyLinux
 * Endless OS (Debian Stable)
 * Tails (Debian Stable)
 * Q4OS (Debian Stable)
 * PureOS
 * SteamOS (Debian Stable)
 * Corel Linux (Discontinued)
 * KNOPPIX
 * AV Linux
 * Kali Linux (Debian Testing)
 * Parrot Security (Debian Testing)
 * Deepin
 * UOS

Ubuntu and derivatives

 * Ubuntu
 * Kubuntu
 * Xubuntu
 * Lubuntu
 * Edubuntu
 * Linux Mint
 * Ubuntu Studio
 * Mythbuntu
 * elementary OS
 * Linuxfx
 * Peppermint Linux
 * Linspire
 * Zorin OS
 * Linux Lite
 * Ubuntu Kylin
 * Ubuntu Budgie
 * Pop!_OS
 * Ubuntu MATE
 * Bodhi Linux
 * Robolinux
 * Ubuntu Touch

Fedora, Red Hat and derivatives

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

Others

 * PartedMagic
 * Slackware
 * Arch Linux
 * Damn Small Linux
 * Gentoo
 * openSUSE
 * GNOME OS
 * Mandriva Linux
 * Gecko
 * Manjaro
 * Void Linux
 * Tiny Core Linux
 * Android
 * Chrome OS
 * Kogaion
 * Vine 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 mentioned when they were about to select the mascot that he was bitten by a penguin one time on a trip to a zoo in Australia. This fact became the inspiration for Linux's mascot.