There are four Clans in the forest and one "lost" Clan. The forest Clans are, ThunderClan, RiverClan, WindClan, and ShadowClan. The "lost" Clan is SkyClan. The Clan that almost every book is about is ThunderClan. ThunderClan is a Clan that lives in a dense forest. RiverClan is a Clan where it is an island. These cats have to be good at swimming! ShadowClan is a Clan where they live in a dark pine forest. WindClan is an open moor. These cats are fast because they eat rabbits. Now the "forgotten" Clan, SkyClan, is a Clan made up of former kittypets and loners some of these cats still live with Twolegs.
There is also StarClan, a Clan that cats go when they die. Not all cats come here. Cats like Tigerstar, Darkstripe, Clawface, and Brokenstar, did not go here when they died. They went to the Dark Forest. The Dark Forest is where evil cats go. In Omen of the Stars, the Dark Forest warriors trained living cats in their dreams. StarClan sends mysterious omens to Clan leaders and medicine cats to help with decision and for warnings.