Game demo
A game demo is a freely distributed demonstration or preview of an upcoming or recently released computer or video game.
Demos are typically released by the game's publisher to help consumers get a feel of the game before deciding whether to buy the full version. For console video games, they are often released with magazines that include the demos on a CD or DVD and likewise may be exclusive to a certain publication. Demos are also sometimes released on cover tape/disks, especially in the United Kingdom and mainland Europe, but given the increasing size of demos and widespread availability of broadband internet, this common practice throughout the 1980s and '90s gradually lost cover focus to full games. With the advent of console online services such as Xbox Live, demos are also becoming available as a free or premium download.
Console manufacturers also often release their systems with a demo disc containing playable previews of games to be released for their console.
Distribution
In the early 1990s, shareware distribution was a popular method of publishing games for smaller developers, including then-fledgling companies such as Apogee Software (now 3D Realms), Epic Megagames (now Epic Games), and id Software. It gave consumers the chance to try a trial portion of the game, usually restricted to the game's complete first section or "episode", before purchasing the rest of the adventure. Racks of games on single 5 1/4" and later 3.5" floppy disks were common in many stores, often very cheaply. Since the shareware versions were essentially free, the cost only needed to cover the disk and minimal packaging. Sometimes, the demo disks were packaged within the box of another game by the same company. As the increasing size of games in the mid-90s made them impractical to fit on floppies, and retail publishers and developers began to earnestly mimic the practice, shareware games were replaced by shorter demos that were either distributed free on CDs with gaming magazines or as free downloads over the Internet, in some cases becoming exclusive content for specific websites.
Shareware was also the distribution method of choice of early modern first-person shooters (FPS) like Wolfenstein 3D and Doom. Following Doom, as game publishers picked up on the practice of offering demos, third-party distributed shareware declined over the rest of the decade.
The availability of demos varies between formats. Systems that use cartridges have typically not had demos available to them, due to the cost of duplication, whereas systems supporting more cheaply produced media, such as tapes, floppy disks, and later CD-ROM and DVD-ROM have. Also, the internet has more recently been a source for demos, although typically this is in addition to other distribution media available for the system in question.
Types
Game demos come in two variations: playable and non-playable (also called a "rolling demo"). Playable demos generally have the exact same gameplay as the upcoming full game, although game advancement is usually limited to a certain point, and occasionally some advanced features might be disabled. A non-playable demo is essentially the gaming equivalent of a teaser trailer.
Playable
Generally, playable demos are stripped down versions of the full game, restricting gameplay to some levels, only allowing access to some features, or limiting the amount of time playable in the game.
However, some demos provide content not available in the full game. An example of this was the Age of Empires demo which included a Hittites campaign and two maps not available in the full version. Also, the Half-Life demo Half-Life: Uplink is a self-contained game, adapted from material cut from the development of the main game.
In other cases a demo may differ from the equivalent section in the full game, for instance when the demo is released as a preview before the full game is completed.
Demos for platform or other action games generally only include the first few levels of the game. Demos of adventure games are often limited to a very small number of rooms, and have the "save game" feature disabled. Demos of sports games usually limit play to an accelerated half-time or complete match between a small number of teams (which at the same time led to the practice of "demo expanders" that allow the tweaking of some of those settings). Likewise, demos of racing games are ordinarily restricted to a single race with a pre-selected car.
Non-playable
A non-playable demo is a recording of gameplay, either recorded in a video, or played through using the game's own engine. They are mainly displayed at gaming conventions, such as E3, when the game is still in early production as a technology or gameplay preview. Such demos might also be distributed through the Internet or with magazines as trailers for an upcoming game, or featured at retail stores (often among playable demos).
Players who wish to show off their skills or some feature in a game might record a demo of their progress. Magazines or websites often challenge readers to reach certain tasks in a game (such as reaching the highest score, or the quickest time of completion), such as the case of speedrunning.
Some games show demos when the player idles on the title screen for a short period of time.