In the Warriors universe, a large group of feral cats has settled into a forest and claimed it for their own. The cats are split into four smaller Clans: ThunderClan lives in woodland areas, WindClan resides on the moors, RiverClan by the lake, and ShadowClan within and around the swamps hidden in the shadows. SkyClan, a fifth group, also used to be a part of this system but was forced to flee when their territory was destroyed to make human houses. The other Clans refused to share their territory, thus leading to SkyClan being driven out of their home. The other Clans were ashamed of this and tried to erase all memory of SkyClan. Each Clan has adapted to their own terrain; RiverClan, for example, will swim in the river to catch fish, while the majority of cats from the other Clans fear and avoid bodies of water.
Relationships between different Clans are tense and they often come into conflict with one another. Each clan is taught signature battle moves in order to protect their territory. Border skirmishes are the most common source of battle; they are often short-lived and cause no real damage to either side. Bigger and more brutal battles can occur, which can result in casualties. The Clans can also show concern for each other; the idea of one Clan being destroyed causes deep distress and will prompt urgent action on behalf of all four Clans. On the night of every full moon, the Clans will meet at a designated spot in order to share stories and news in peace. This is known as a 'Gathering', and fighting is prohibited.
Cats who live outside of the Clans are split into three groups. Housecats, known as 'kittypets', are often sneered at by the Clan cats for their cozy and lazy life, and if they wander into the forest are chased away, but will rarely come to harm. 'Loners' are cats who walk outside of the Clans, usually by themselves. Although regarded by the Clans with suspicion, they do not cause the forest cats any trouble, and so are free to wander outside of the borders in peace. Similar to loners are 'rogues', who differ only because they try to cause Clans harm. It is not unusual for a rogue to be a former Clan cat looking for revenge. For a kittypet, loner, or rogue to join a Clan is rare, but not unheard of. Often, these newcomers have to prove themselves in order to be regarded with trust by the rest of the community.
Beyond the Clans' territories lies a mountain range, inhabited by the Tribe of Rushing Water, which follows a different set of ancestors: the Tribe of Endless Hunting. The Tribe has a Healer, cave-guards, and prey-hunters, who each serve a different function in the Tribe. The Healer leads the Tribe, heals the ill and wounded, and communicates with the Tribe of Endless Hunting; the cave-guards defend the Tribe and the prey-hunters hunt. The Tribe was formed by the Ancients when they left the lake to live in the mountains. In turn, cats from the Tribe moved to the forest of the original series and formed the Clans.
StarClan is a group of the Clans' deceased ancestors who give guidance to the Clans. After death, most Clan cats join StarClan. It is represented by Silverpelt, and each individual star represents the spirit of a single dead warrior. Upon joining StarClan, the cats' spirits take the form in which they were most happy while living (i.e. blindness and deafness can be cured). StarClan Warriors, elders, deputies, queens, and kits keep watch over the Clans, usually watching the Clan they lived in while alive. They provide guidance to the Clans, often through dreams and other signs like omens. Often, this occurs when medicine cats go to the Moonstone, a large piece of quartz in an abandoned mine, which is used in the forest territory to communicate with the medicine and leader cats' ancestors every half-moon. StarClan cats are often described as being transparent and silver in color, with stars sparkling around their pelts.
In addition to StarClan, there exists the Dark Forest, also known as The Place of No Stars. The spirits of traitors who caused great pain and suffering to others walk alone there, forever, to pay for their sins. As the name suggests, it takes the form of a never-ending forest, forcing its residents to forever walk alone. Although cats of the Dark Forest are meant to be isolated as punishment, they later learn how to walk in the dreams of living cats. Like StarClan cats, Dark Forest residents will fade away if they are forgotten, or killed in battle.
The Warriors universe setting has been used for several series of novels and is presented as the default setting for Warriors Adventure Game.