Linux is a Unix-like operating system created as a free software alternative to other Unix-like operating systems. The core component of any Linux-based operating system is the kernel, which was originally authored by Linus Torvalds and first released in 1991. Linux is typically distributed with user space applications developed from the GNU Project. Because of this, the Free Software Foundation, which created the GNU Project, prefers to refer to the operating system resulting from the combined kernel and applications as GNU/Linux.
Linux is generally released in a distribution, such as Ubuntu, Gentoo, Debian, or Fedora (though a skilled user could theoretically build it from scratch). These distributions can be commercial, like Red Hat, or free like Ubuntu.
Developer Linus Torvalds created Linux in 1991 due to dissatisfaction with the license restrictions of the MINIX operating system, which only allowed distribution for educational purposes. He initially developed the Linux kernel on MINIX and MINIX applications were used along with it. Development eventually moved to Linux with a GNU user space when the kernel had matured.
Torvalds initially released Linux under a license that restricted commercial distribution, but in 1992, the license was switched to the GNU General Public License (GPL). The GPL is a free software license and does not contain the same restrictions as Torvalds' initial license choice. After switching the license to GPL, developers worked on porting the GNU Project applications to Linux, creating what is now commonly referred to as Linux or GNU/Linux.
Linux has long been a popular choice for free and open source games like NetHack. Commercial games started appearing in 1994, when ID Software's Dave Taylor ported Doom to the platform. With the creation of Loki Software in 1998 porting became more common. Former Loki Software programmer Ryan C. Gordon continues to be involved in porting Linux games to this day.
Windows compatibility layers such as Wine can be used to play games native to Windows on Linux, though it's not typically the best way to play those games. Wine can, however, run the Windows version of the Steam platform on Linux, which has a much greater library than the Linux version. The Wine website has a detailed database of games with information on how to run them in Wine.
Indie developers generally support native Linux versions much more than larger publishers, with games like Minecraft being very successful on the platform.
The Humble Bundle debuted in 2010, offering a selection of games to raise money for charity. Games are available for Windows, Mac and Linux, sometimes with games making their Linux debut as part of the bundle. Typical sales for the Linux version of the bundle rank it third, but the Linux users generally pay the highest average amount of money for the bundle(Contributors pay what they want).
In 2013 the Steam digital distribution platform was officially released for Linux along with a variety of games. Although Valve only officially supports this version of Steam on Ubuntu, it can be installed on other distributions. Valve has also confirmed that their version of the "Steam-box" will run a version of Linux, though users will be able to install other OS's.
On July 24, 2014, GOG.com officially started supporting Linux (four months after announcing their plans to add Linux support), inititally with a batch of 50 games, with another hundred to be made available in the following months. The games are made sure to work in the Ubuntu and Mint distributions and includes .deb installers for ease of use on those (and other Debian derivatives), but also include distro-independent tar.gz files.