ThunderClan is a group of cats that live in a part of the forest that is primarily deciduous woodland. The founding leader, Thunder, got his name from the Thunderpath, so by extension the name was thought to be given to the Clan.
ShadowClan is a group of cats that live in the swamplands of the forest. Their founding leader, Shadow, had gotten her name from the silent way she hunted at night, so the extension of the name was thought to be given to the Clan.
RiverClan is a group of cats that live in a part of the forest that is primarily wetland, or shores of lakes/rivers. The founding leader, River, got his name from moving through the forest like the graceful river, so by extension the name was thought to be given to the Clan.
WindClan is a group of cats that live primarily on the open moors. The founding leader, Wind, got her name from being able to run as fast as the wind, so by extension the name was thought to be given to the Clan.
SkyClan is a group of cats that live in a more open forest, with high trees. The founding leader, Sky, got her name from being able to jump far and high, so by extension the name was thought to be given to the Clan.
StarClan are the deceased warrior ancestors of the Clans who live on in spirit form after dying, and watch over the living Clan cats. There is no specific leader for StarClan, since there are many cats from all four Clans that work together. They are responsible for sending signs and omens to the living cats.
The Dark Forest is the opposite of StarClan's hunting grounds. It is a place where cats that have committed great crimes against their Clans and against the Warrior Code go when they die. However, cats who commit crimes but believe they are following the Warrior Code by doing them can usually join the ranks of StarClan instead