The Clans are, in simplest terms, groups of cats that live in the woods and have to fight, hunt and sometimes work together to survive. Kittypets (house cats) and rogues (strays or cats outside of the Clans) are usually frowned upon by the Clan cats. In some cases these cats have joined the Clans, but they are often teased for their birth origin (ex. “Gross, you’re a Kittypet! You’re not Clanborn!”)
There are ranks within these Clans, from the top to bottom, Leader, Deputy, the Warriors, the Queens, the Apprentices, and finally, the Kits. At six moons old (six months) a kit becomes an apprentice. Let’s say the kit’s name is Bluekit.
They go through a ceremony and become an apprentice, then they are given a mentor. The apprentice ceremony renames the kit Bluepaw, the suffix -paw meaning an apprentice.
After moons of training, I believe it was maybe ten moons (ten months) the cat can go through an “assessment” so basically their mentor watches them from afar, seeing how their hunting skills are and stuff such as. If the mentor tells the leader that they did well and passed, then they can become a warrior. The apprentice, Bluepaw, receives their warrior name, a suffix that usually has something to do with a physical characteristic (ex. Good at hunting: Blueclaw) or personality trait (ex. Blueheart).
The warriors hunt and fight, the deputy organizes the patrols for hunting and checking the borders for enemies and scent markers that have faded. If a cat becomes a deputy their name does not change.
If a cat becomes leader, which usually only happens if the former leader has died, they become (ex. Bluestar). Queens raise the kits.
StarClan is the ‘religion’ of these cats, it is where the good cats who haven’t broken the warrior code go when they die, bad cats go to the dark forest. I’m not sure where the atheistic cats go. It’s basically like Valhalla and Hell but for cats. StarClan is like an endless hunting grounds where the prey is plump and juicy and there are perfect places to rest in the sun or shade.
The Dark Forest is a pretty apt name; It’s dark, the water there is a murky muddy abyss, and everyone either teams up with the strong cats or gets killed off. If a cat in either StarClan or the Dark Forest dies a second time, they cease to exist, they are not reincarnated but literally just fade away into nothingness. This also happens if a cat is forgotten about by the living, so everyone eventually fades away but this normally takes a long time.