The Clans are "human" in their origin. The Warrior Caste has been genetically engineered to be the "cream of the crop" from the DNA of previous Clan Warriors that prove themselves in combat and honorable actions.
There is far more detail to that, which can be found in reading about the Clans.
A general over view of the Clans way of life -
The Clans were founded off an idealology.
Kerensky as mentioned previously saw the squabbling of the Houses and other politics at play and made the choice to find those devoted to him and his cause and acquire as many ships and materials, personnal as he could, and when the fighting started ti engulph the InnerSphere. They left known/charted space and headed to the unknown (there are rumors they he had scouting parties that were charting space beyond the Innersphere.)
Kerensky's armadda experienced their own division on at least two occasions before finding the "Cluster worlds" which would become the home of the Clans.
Not to say that Kerensky and his fleet would not see their own internal strife but not nearly as bad as the IS saw over the same timeline.
Eventually the Eugenics program started and they cultivated what would become the class system. Warriors, bandit, Scientist and a few others.
With the exception if the Warrior Class, the other classes can go about life as we know it today, with "freebirth", everday normalities. Albeit under Warrior caste rule in which if one wants to pursue politics, and rank in the society that is usually restricted to the warrior caste.
Warriors can fail out of training and relagated to a lower caste. Warrior cadets are killed in training as well.
Though HIGHLY grown upon, Warrior caste does interact with the lesser classe and 'free born" kids have come from those relationships.
On the opposite end of the spectrum. A free birth can if allowed and proves worth join the warrior caste but it is a difficult and dangerous process.
As there are right wing and left wing politics and beliefs in our society there are the Warden and Crusaders of the Clans awith varying degrees.
What makes the Clans unique is that for the most part the Warrior caste is "test tube" babies and they have what are called Sibkos. Think of it as your "brothers and sisters" that you are vat born, raised, educated, fight with, train with and yes even sleep with (They are born sterile usually) though pregnancies and birthings do happen. In that regard the writers controlled the evolution of the Clan Warrior caste and it's science but also allow for the will of mother nature to play into the stories of characters.
There are many rights and codes that teh Warrior Class can invoke and how they live their daily lives. Ideally honor in all actions and thoughts but as we all know, being human, even if genetically altered we are prone to error, misjudgment and action based on emotion, rather then logic.
The Clans aim to weed out the weak and promote the strong, however the weak are needed for the strong to thrive.
Much like the Marine corp has approximately 200K compared to Army/Navy and Airforce, then the civilian pop. The Clan warrior caste actually makes up a small % of the Clan populace, however they weild much power in politics and government.
The vision of Kerensky was to leave the IS. Cultivate a more ideal sociey then return when the houses had whittled tehmselves down, and then take control of terra.
So in that regard as far as the IS is concerned they are the Enemy. As far as the Clans are concerned they are "coming home" to take over.
The difference within the Clans themselves is as I mentioned the Wardens and the Crusaders one feels teh Clans can incorporate with the IS and improve the way of life and actually return home, vs the other side wishes to take over the IS and force Clan way of life on the IS.
Due to the drastic difference in way of life between the Clans and IS there is a divide that when one steps back and looks and takes in teh whole picture, really we aren't that different. Yet as mankind has done for thousands of years. We are constantly fighting wtih ourselves and that is clearly shown in the conflicts with in the houses, and the IS vs the Clans because each side thinks it's way of life is more ideal. As is the case with our world today.
Okay so that turned into WAY more then I wanted to write........but I tried for a very general description of WHY and WHO the Clans are.