There are currently four clans available to join (should you meet their requirements). Joining is permanent and gives you benefits such as bonuses to attack, armor, health, etc. as well as a recognized post-title signifying your clan membership.
Each location is hidden somewhat, but I'll give you some hints and some insight into the specific benefits of each clan:
BloodJaw Clan This clan is out in the Sal va'Taer Forest on the logging trail between Semanri and Tes'kata Village. The clan leader Andu trains members to withstand endurance trials and as such, all members gain a moderate bonus to their maximum health and a significant bonus to armor rating.
FrozenHeart Clan Well-hidden near the base of Mount Lexunaze, this small clan is led by Drabola who trains her members to protect themselves first and strike next. As a result, members gain a moderate bonus to armor rating and a significant bonus to attack rating.
SteelBand Clan Out in the desert trails of the Loup'Garou Desert south of Cal'Ziz, the SteelBand clan is another small group of members who are taught with simular respects to the teachings of clan leader Drabola. This clan's leader, Hallom, focuses primarily on combat power while keeping up one's guard. Members gain a moderate bonus to attack rating and a significant bonus to armor rating.
GoreHunger Clan Probably the most feared clan, GoreHunger members are taught by the ferocious Wernvol, who instructs his members to be well-balanced and that only through mastering everything can you truly be powerful. GoreHunger has their camp based almost immediately outside the eastern entrance to the Crystal Caverns. Members gain significant bonuses to attack rating, armor rating, and maximum health.
Please discuss this announcement in the Barbarian forum should you have quetions, concerns, ideas, etc.