Ryzom is a massively multiplayer online role-playing game (MMORPG) developed by the independent French studio Nevrax. The game launched in September 2004 under the name `The Saga of Ryzom` and received little attention at the time. In August 2006, the name was simplified to `Ryzom` for marketing purposes. Ryzom is labeled as Science-Fantasy by the creators due to storyline elements that are typically found in both science fiction and fantasy genres.

The game takes place on the planet Atys. Unlike normal planets, Atys is an enormous tree large enough to sustain an atmosphere and multiple ecosystems on its surface and within immense networks of branches and roots. Four distinct humanoid races, or homins, came to exist on Atys hundreds of years prior to the game`s current timeline (although it is not specified how or why) and developed into separate, fairly isolated societies in their own favored ecosystems. Interacting benevolently with all four homin cultures, but hostile towards each other, were two additional and far more advanced races: the magical Kami and the technological Karavan. The Kami are depicted as elusive, chaotic, playful, and highly protective of Atys, while the Karavan are more secretive and have never been shown outside their environmental suits or far from their machinery.

In the game year 2481, the homin races were decimated when the hostile, insect-like Kitins were accidentally released from their home deep within Atys`s root system. Surviving refugees from all four homin cultures began working together in 2485 to rebuild a single, mixed society in relatively remote areas. Soon came the added threat of the “Goo,” a spreading infestation that renders areas uninhabitable and drives wildlife mad with rage. At release, the game year was 2525.

The Nevrax team has striven to make Atys simulate a realistic environment. Various creatures have differing AI based upon their species that allows them to perform many realistic behaviors, such as migration and movement as a herd. Carnivores, for example, will attack specific other animals for food, and certain animals hunt in packs. The game also features changing seasons and weather effects. Weather effects include rain, snow, and wind. Tied to the season and weather changes are the movements of animals and availability of harvestable materials. For example, a certain type of sap may only be harvested during spring rains and be unattainable during other seasons or weather conditions. Weather conditions can change minute by minute. Each game season (spring, summer, fall, and winter) lasts four realtime days.

In many other MMORPGs, each character has a single class, and predetemined benefits and new abilities are gained based on experience points. Ryzom has no classes and no overall character levels. Instead, each character gains levels independently in the fields of weapon-oriented fighting, magic, crafting, and harvesting simply by performing appropriate acts (killing enemies with a sword for fighting, assembling jewelry for crafting, etc.). Each level gained in one of these fields awards the character 10 skill points that can be spent on new abilities specific to that field, spent on general character improvements, or simply saved. All abilities are built of components called stanzas. Each stanza defines one of the ability`s effects, costs, or usage restrictions. Players may assemble actions out of any legal combination of stanzas their character knows. For example, if a player wanted to make an attack that strikes all enemies which are engaged in melee with a circular weapon swing, causes extra damage, and also leaves its victims bleeding, he would assemble the Circular Attack stanza, an Increase Damage stanza, and a Bleed stanza. (He would then have to add enough credit and restriction stanzas to balance the beneficial effects, such as a high Stamina cost to perform the action, or a moderate cost but the limitation that it can only be used after parrying an enemy blow).