Playable
demos are one of the most useful tools for publicising a
video game. The concept is broadly similar to the
music industry mainstay of releasing
singles (and in recent decades,
promotional videos) with the intent of encouraging consumers to buy the
albums from which they're taken. The term 'playable demo' usually refers to a
portion of the game (typically a single level or sequence) released
gratis to the general public (as opposed to pre-release demonstration versions either shown at
trade shows or sent to the press). Playable demos have been popular for
home computer games since the
8-bit era, and with the move from
cartridge to
disc technology they have been adopted by the
console sector also.
The main methods of distribution for playable demos are specialist magazine covermounts (which have gone from cassette tape and floppy disc to CD-ROM and DVD-ROM), and the internet (via games news sites). Both of these methods are hampered to an extent by the increasing size of game assets, leading to demos weighing in at hundreds of megabytes vying for a finite amount of disc space and bandwidth, but they have the benefit of being virtually free for the publisher (the relevant media outlets pay for hosting and disc duplication). Occasionally demo discs are given away at retail outlets, or are packaged with other games by the publisher. (The most successful use of this strategy being the inclusion of a demo version of Metal Gear Solid 2 with the game Zone of The Enders, which prompted many gamers to purchase the game simply to play the demo.)
The playable demo's greatest strength is (obviously...) that it's playable. The experience of playing a game cannot be adequately conveyed through purely passive video footage or still screenshots. A playable demo will give the user some idea of how the controls function and what themes and elements are emphasised in the gameplay. They can also explore the environment and assess the production values up close and (for PC games) gauge roughly how the game will look and move on their hardware.
There are some hazards that the developer must be wary of when constructing a playable demo. The first is that they must ensure that the demo provides an accurate and positive representation of the full game. To this end it is preferable that the game technology has been tested thoroughly by the time the demo is released, as a slow or buggy demo will show the game in a bad light. (See the first version of the demo of Splinter Cell that was released on PC and Xbox.)
The content of the demo is also important. An easy trap to fall into is to give the player the earliest section of the game, which might be very easy and uneventful compared to later sections where the player is assumed to have gotten to grips with the controls and conventions of the game world. (This mistake was recently made by Pom Pom, who initially released a demo of their game Mutant Storm which contained only the first five stages of a 70+ level game. They later released a demo with levels hand-picked from various points in the game.) The developer must be careful not to include too much (offering a mode that is extensively replayable, even if limited in features, might actively discourage players from shelling out for the full game) or too little. A few demos (such as Half Life: Uplink) consist of levels that are not included in the full game- the 'new' content is usually (but not always) made up from levels that have been cut from the full game. This kind of demo might be given to one magazine for exclusive distribution.
In the context of modern games, playable demos are now seen as being expensive and time consuming to produce, as they require an appreciable coding effort to isolate a subset of elements from the full game, as well as quality assurance testing (and even in some recent cases, subsequent patching!). As a result many developers will hold off building and releasing a demo until long after the full game has reached the shops. In other cases the developers forego releasing a demo at all, instead choosing to release non-interactive video footage. Sometimes (PC) developers will release a public beta in lieu of a demo, although these generally cannot be distributed on magazine disks.
A similar method of promotion to playable demos is the shareware model, although this has fallen out of favour in the (heavily commercialised) games sector. (Although in the first half of the 1990's shareware was championed on the PC by Apogee, Id Software, Epic MegaGames and on the Atari ST by Llamasoft.)