An interest in playing older video games. Retro games have had an upswing in interest in the last 10 years as the average gamer gets older, and computers have become sufficiently powerful to emulate earlier systems without affecting the gameplay or graphics.
Retro gaming generally plays out in a 2-D game space, usually offering gameplay with no determined conclusion other than loss of all lives, often using a simple numerical score or just "getting to the finish" as its game goal. Controls are likely to be simple and non-analog with only one or two fire buttons. For this reason retro gaming has recently become more prominent on mobile phones where these attributes are advantageous to the platform, a platform which encourages simple gaming experiences that are quick to learn, use simple controls and are easy to pick up and put down.
Retro gaming, being the infancy of video gaming, was the start of many many gaming tropes and genres. Shoot 'Em Up, Beat 'em Up, Platform Game, Puzzle Game... virtually every genre started here, including 3D gaming. In these times, the hardware was a much more visible limitation of what the games could do, including colour palette, number of things on-screen and even the sound. This practice is where All There in the Manual gets its name, where the characters, story and even gameplay mechanics would be described in an instruction manual because it was difficult to do this within the game itself. This is also where the "Console Wars" were born as kids argued over which system was better or worse, a practice that continues to this day.
The first perennial question for retro gaming is, "What counts as retro?". Diehard older retro gamers may insist that only pre-crash games and systems count, while more liberal definitions have a moving point of retro as any system at least 10 years old (which itself is debated considering modern systems can last that long all on their own). Most people seem to pin their preferred retro gaming system to "whatever I was playing when I was 12". For some, Retraux Gaming also counts.
The second perennial question for retro gamers is "real hardware versus emulators", generally a question of authentically playing old games on their original hardware or having them run on more modern systems at the cost of accuracy, with pros and cons for each. For original hardware, plugging an old console into a modern TV can add significant lag due to image processing, and may appear especially poor for systems from the mid-90s or earlier, so for some, "original hardware" extends to old CRTs to play them on, while others may consider buying a device like a RetroTINK or the OSSC to improve the picture quality. Buying old consoles and games is also very expensive, and the condition these will be in can affect well how they run, or at all (not to mention where you'll put them). But the upshot is that you'll be playing the games as they were intended, and they'll run, look, feel and play exactly as they when they were created, and for many people that's incredibly important.
Emulation on the other hand means taking these games outside of those contexts. For example, some systems relied on effects caused by the fuzziness inherent in their output to allow the illusion of more colours on screen than was strictly possible for the hardware, and the CRTs they expected to run on had rectangular pixels rather than the square ones of today, meaning that an emulator may not show a retro video game as the makers intended. Some very old systems are nearly impossible to emulate, as they were based on analog hardware, or in the homebrew scene may run on a console that hasn't been emulated at the moment.
For example, for someone who grew up with a computer with a particular sound card,note not having the sound card emulated in DOSBox is a good enough reason to go and splash money on the hardware route for some because "the game will not sound like how I remembered it".note Also, games may not work as well in emulation, due to various factors that can contribute to input/output latency;note this is often made worse by attempts to sync the screen refresh to that of the monitor in order to avoid tearing and juddering. The upshot however is convenience: this method is purely digital, you have a vast array of games available at your fingertips, they're free, and the games are so small that a new one is only an instant download away. They can also be used to make the games look better than they did, like render Spyro the Dragon in HD, widescreen and a higher colour depth than was possible on the original hardware.
Emulation is also a common way for publishers to re-release their older back-catalogue on modern platforms: take a shot every time Sonic the Hedgehog 2 is re-released this way. While they can also run the risk of the same problems and inaccuracies as "homebrew" emulation, they are more accessible with no legal grey area, and run on systems you can plug into your HDTV with minimal worries. A third option is FPGA consoles, which can exactly copy the way the original hardware worked for extremely accurate replication while running the original carts, while also allowing for digital ROMs and modern conveniences like HDMI output.
It is retro gaming that often supplies the sound effects in television and movies to denote that video games are being played, regardless of the fact that Halo does not sound remotely like Galaga or Pac-Man. While very outdated and often full of Narm, this works because that soundscape is exclusive to video games, and so is highly unlikely to be confused with anything else.
Home of the Underdogs is a 'digital museum' of underappreciated games, the vast majority of which are fifteen to twenty years old. (Its previous incarnation at http://www.underdogs.net/info became defunct a few years ago due to an apparent lack of funding.) The Retronauts podcast (formerly of 1up.com) is dedicated to discussion of retro gaming.
There are now museums and organizations set up around the world dedicated to capturing the history and examples of the early years of videogaming, such as the Video Game History Foundation. Much like early Doctor Who tapes, video game development was thought of as disposable, and little to no effort was made to conserve source code, original artwork and other associated products of the process, with games as recent as 2002's Kingdom Hearts having all its development resources lost (something that still happens today). Further, early (and not so early) storage media are notoriously unreliable, and efforts are being made to transfer early digital artifacts into more stable form.
In the mid-New Tens, the Internet Archive began several projects to preserve early video games in danger of being lost. These projects, which allow for in-browser emulation, include the Internet Arcade for arcade games and archives of MS-DOS and Windows 3.x games.