The Hyuuga clan is one of the truly noble clans of Konoha village. It is a highly respected clan, and well-known for its advanced bloodline technique of Byakugan (also known as White Eyes or Evil Eyes). The Hyuuga clan consists of two families, the main family and the branch family. The lower, branch family is put into place to serve and protect the main family, and has done so for generations. The reasons behind this separation is because the main family is destined to hold the original bloodline of the Hyuuga clan, resulting in the absolute need to protect it. Of course this resulted in the branch family being formed to simply sacrifice itself to protect the main family.

From a young age, the members of the branch family are possessed with a terrible curse in the form of a mark on the forehead, which allows anyone in the main family be able to control and manipulate them. All that a member of the main family would need to do is form a simple hand seal which inflicts severe pain on the member of the branch family, and is used if the member were to ever step out of line. The curse seal Jutsu that is cast on them is displayed by a symbol of a "Bird in the Cage" (the Swastika which was altered for the anime version as seen in the image below) which is found marked on their forehead until the moment of death.

Of course, above all, the Hyuuga clan is known for its bloodline and the ability of "Byakugan". This ability gives the user an 359° vision with the resulting 1° being the only weakness of the "Byakugan". Having an almost 360° vision gives the user an absolute defense which of course makes the Hyuuga clan such an asset to Konoha and certainly makes them on par with the Uchiha Clan which of course is also known for it's eye technique. In combat terms the Byakugan isn't just useful in defense, it's highly destructive when used offensively.

Such techniques as "Heavenly Spin" and the preferred style of "Hakkeshou" (the Chinese martial art of gentle fist fighting) are heavily used by the Hyuuga clan in combat. Byakugan also gives the user the ability to visually see the flow of chakra that runs through an opponent's body, with this the Hyuuga clan can use the key move of "64 hands of Hakke". When done successively can block every single chakra opening on the opponent's body resulting in the opponent being unable to use any Jutsu's, but "64 hands of Hakke" demands an extreme physical capability by the user.

Referrence (http://www.naruto2.com/Information/Clans_Guide/Naruto-ClansGuide_Hyuuga.aspx)