A clan is a collection of players coming together under one banner, united under one cause.
Since all civilized lands in the realm are owned by the rulers of the kingdoms. In the tradition of vassalage, most clans pledge loyalty to their king and by way of that loyalty or some birthright they come to own land upon which they build their home.
In rare cases a clan may choose to remain freemen, not bound by oath to any ruler. However, because they pledge no loyalty to a king they cannot own their own land. They are considered outlaws since they take from the land without the blessings of its ruler and their lives are often fraught with danger.
Only the Mages of the Arcane exists in peace with the kingdoms while owing them no allegiance.
As each kingdom has its own rules of conduct so does each clan. Thought they have their own goals though these goals must usually coincide with those of their kingdom.
You must be letgained to join a clan then must petition a member of the clan you wish to join. Remember that joining a clan is a dangerous step, you will be required to uphold the oaths you have taken and fight to protect both your clan and your kingdom.