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 where there in a dense forest. RiverClan is a Clan where its an island these cats have to be good at swiming! ShadowClan is a Clan where there living deep into the forest. WindClan is an open moor these cats are fast becuase 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 come here when they died. They came to the Dark Forest. The Dark Forest is where evil cats go. In the Omen of the Stars seiries the Dark Forest warriors train living cats in their dreams. StarClan sends mysterious omens to Clan leaders and medicine cats to help with desicions and for warnings.