A clan is a large family/kinship system from scotland in which weatlhy/powerful families, and those who married into that family group, settled and ruled the country side. A clan hand lands that provided homes and fields to work for tenents and homesteads for family members. Usually there was a castle where the head of the clan and his immediate family lived. This was the focal point for all justice, legal, and large gathering activity was done. Since families married in and out of the acctual name of the clan it is easy to have a different last name from that of your clan. The clans were almost wiped out after the Rising at Culloden in 1745. The names existed, but the power and control of lands was stripped away by the English in the years following the rising. You don't just get into a clan, you are desended from a clan. My family name is no where near MacDonald, but it is traceable back to Clan MacDonald.
Hopefully that made sense, I'm tired and probably babbling.
"All the while making faces like a baby platypus who forget to take some Beano before eating a chimichanga." FindingNamo in reference to Jessica Simpson's singing.