Linux

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.

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.

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, 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)
 * Pardus
 * Deepin
 * UOS

Ubuntu, 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, derivatives

 * Red Hat Linux
 * Fedora
 * Red Hat Enterprise Linux
 * CentOS
 * Rocky Linux
 * 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
 * OLPC OS
 * SME Server
 * Qubes OS
 * Miracle Linux

Other derivatives

 * Parted Magic
 * Slackware
 * Arch Linux
 * Damn Small Linux
 * Gentoo
 * openSUSE
 * Gnome OS
 * Mandriva Linux
 * Gecko
 * Manjaro
 * Void Linux
 * Tiny Core Linux
 * Android
 * Chrome OS
 * Kogaion

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.