Warrior Cats is a series written by Erin Hunter. It has spanned over several years with multiple series and a handful of main characters. There's at least one favourite cat for everyone! The first book, Into the Wild, introduced the first main character, Firestar. We followed his life from kittypet to apprentice all the way up to leader, and have seen a whole world bloom around his adventures.

CLAN STRUCTURE

Much like our forum here, there are four clans residing in his forest; ThunderClan, RiverClan, WindClan, and ShadowClan. The four clans follow StarClan's teachings, attend meetings upon the full moon, and are fiercely loyal to their companions. These clans follow a general hierarchy which looks much like this;

Leader -- Name will always be followed by star [e.g. Firestar]. The leader is granted 9 lives upon acceptance by StarClan.
Deputy -- Organizes patrols of all kinds and is generally picked by the leader. Takes leaders place when they step down / lose all lives.
Medicine Cat -- Focuses on healing their clanmates. Generally very knowledgeable on herbs. Can be multiple Medicine Cats.
Medicine Cat Apprentice -- In training to become the next Medicine Cat.
Warriors -- Adult cats who have proven themselves worthy to their clan. 
Apprentices -- Young cats older than 6 moons. Name followed by paw until made a Warrior. [e.g. Silverpaw]
Queens -- A she-cat who is either expecting or has had kits. Even without kits, some she-cats stay Queens. 
Kits -- Cats younger than 6 moons. Not allowed outside of camp until Apprentices. Name followed by until made an Apprentice. [e.g. Silverkit]
Elders -- Adult cats who are unable to perform Warrior duties any longer. Well respected.

Clans are usually tight-knit communities with the desire to protect their clanmates. They are generally hostile towards outsiders and even to other clans. They perform daily hunting and border patrols, storing their food in place designated as the fresh-kill pile. Cats will sometimes be unable to eat as punishment for  their actions. A sense of community hangs over them and cats may forgo eating if Queens, Kits, or Elders don't quite have enough. 

CLAN CEREMONIES 

A ceremony is generally performed for most "coming of age" moments in a cat's life. These include but are not limited to;

Leadership Ceremony -- Leaders will travel with their Medicine Cat to the Moonpool [Star Falls] to receive their lives. They are often given by cats of importance from their lives.
Warrior Ceremony -- Performed when an Apprentice's mentor deems them ready. They will be given their Warrior name and accepted as a Warrior of their clan.
Apprentice Ceremony -- Kits old enough to begin their training. They will receive a mentor and move from the Nursery. 

Other notable "ceremonies" include the clans meeting on the full moon. A truce is generally called while here, although tensions can and still will be high, and the clans intermingle with each other. Announcements about new Apprentices, Warriors, Kits, and other news are often made on these nights. 
Medicine Cats perform a similar gathering with each other, heading to a sacred place to meet. The Medicine Cat ceremony is performed here and is the only one not done by a Clan Leader.

WARRIOR CODE 

The Warrior Code is a law followed by all four clans. This is a fierce law passed through each generation and is highly respected. There are some exceptions, like traveling to the Moonpool and include additional rules

• Defend your Clan, even with your life. You may have friendships with cats from other Clans, but your loyalty must remain to your Clan, as one day you may meet them in battle.
  
• Do not hunt or trespass on another Clan's territory.
• Elders, queens, and kits must be fed before apprentices and warriors. Unless they have permission, apprentices may not eat until they have hunted to feed the elders. If any warrior or apprentice is sick or injured, they may eat while the elders, queens, and kits are eating.
  
• Prey is killed only to be eaten. Give thanks to StarClan for its life.
• A kit must be at least six moons old to become an apprentice.
  
  
• Newly appointed warriors will keep a silent vigil for one night after receiving their warrior name.
• A cat cannot be made deputy without having mentored at least one apprentice.
• The deputy will become Clan leader when the leader dies, retires or is exiled.
• After the death or retirement of the deputy, the new deputy must be chosen before moonhigh.
• A Gathering of all four Clans is held at the full moon during a truce that lasts for the night. There shall be no fighting among Clans at this time.
  
• Boundaries must be checked and marked daily. Challenge all trespassing cats.
  
• No warrior can neglect a kit in pain or danger, even if the kit is from a different Clan.
  
• The word of the Clan leader is the warrior code.
  
• An honorable warrior does not need to kill other cats to win their battles, unless they are outside the warrior code or if it is necessary for self-defense.
  
• A warrior rejects the soft life of a kittypet.
  
• Each Clan has the right to be proud and independent, but in times of trouble they must forget their boundaries and fight side by side to protect the four. Each Clan must help the others so that no Clan will fall.