The Clans on Felisfire are very useful little things. Each clan cost one orb to create, and basic members can create up to five. It cost one more orb to bump up the size of the Clan.
The main purpose of Clan is for breeding. You can put in up to 15 cats to start with, any species you wish. Clans can also be Public or Private. Meaning, with Puplic anyone can freely join and add/remove their cats at will. With Private you have to approve each member along with the cats they request to join, you can edit the members so they can freely add feli.
The clans tend to have a more positive influence on interactions, therefore increasing chances of baby felis.