The Hyūga clan (日向一族, Hyūga Ichizoku) is one of the oldest and most powerful clans in Konohagakure. All members born into this family possess the Byakugan, a kekkei genkai that gives them extended fields of vision and the ability to see the chakra circulatory system amongst other things.
The clan is separated into two parts, the main house (宗家, Sōke) and the branch house (分家, Bunke), a system that protects the secrets of the Byakugan from outsiders. The main house runs the family while the branch house protects it. The members of the branch house are branded with a cursed seal upon the next heir/heiress of the main house's third birthday. This seal gives the main house members absolute control over the branch members, because they can destroy the branch members' minds with a simple hand gesture, or cause them great pain to punish them. This seal also makes sure the secret of the Hyūga's Byakugan is safe, because it seals the Byakugan after the wearer of the seal dies. Because they are essentially servants, simply because of their birth order or heritage, some branch house members are hostile to those of the main house.
Members of the clan are instructed in the use of the Gentle Fist style, which uses the Byakugan to view the opponent's Chakra Pathway System and deal precise blows to disable and impede the flow of chakra. Due to training in this style, Hyūga clan members tend to have excellent chakra control. This is emphasized in jutsu that utilize this chakra control, as well as the nuances of the Gentle Fist style. The clan's strongest techniques, like the Eight Trigrams Sixty-Four Palms, are only allowed to be taught to members of the main house. Some members such as Neji Hyūga, however, have bypassed this restriction by learning them on his own, earning a reputation as a "genius" in the clan.
The Hyūga are the possessors of the renowned dōjutsu - Byakugan. It allows them to view everything in all directions, as well as through most objects. This ability renders the bloodline of the Hyūga to be feared as the 'all seeing eye'. It is said that sneaking up on a skilled Hyūga is almost impossible.
There are, of course, exceptions to their ability to see - which include various sealing barriers and space-time related techniques. There is also a certain limit up to which they can see - which varies from user to user and depends on their skill and practice.