Byakugan

The is a Dōjutsu Kekkei Genkai, a genetic trait, that is shared amongst the members of the Hyūga Clan. Those that inherit the blood of this clan have almost featureless white eyes (Neji's and Hinata's eyes seem to have tints of lavender and blue-grey, respectively). Although it is not stated directly, it is assumed that all members of the Hyūga family possess the Byakugan. It has been said that the Sharingan is a genetic offshoot of the Byakugan.

Just like the Sharingan, the Byakugan can also be transplanted into another person, as Ao of Kirigakure did so. Transplanted Byakugan can be activated and deactivated at the user's will, unlike the Sharingan which is always active.

Their potent abilities make Byakugan highly coveted by the other villages, as evidenced by Kumogakure's attempt to steal it and Kirigakure's efforts to protect their only Byakugan.

Abilities
The Byakugan is the primary focus of the Hyūga's special arts. When the Byakugan is activated, their pupils become more distinct, and the veins near their temples bulge.

The first ability of the Byakugan gives the user a near 360º field of vision, except for one blind spot at the back of the neck above the third thoracic vertebra.

The second ability is having the ability to see through solid objects, and can use a degree of telescopic sight. The distance and the degree of telescopic sight varies from person to person. In Part I, Neji Hyūga was able to see an area of 50 meters, but later he increases his visual range to at least 800 meters, while Hinata Hyūga's range in Part II stretches at least as far as 10 km. During the hunt for either of the Uchiha brothers, Hinata was shown to be able to view Amaterasu's flames, and the chakra in the area, from ten kilometers away. However, the Byakugan cannot see through certain barriers, such as the Four Black Fog Battle Formation used by the Sound Four.

However, the third and one of the greatest uses of the Byakugan is the ability to see chakra, as well as the chakra circulation system inside another's body. It is from this particular ability that they are able to employ their ultimate attack style: Gentle Fist. Unlike the Strong Fist, which is focused on external injuries, the Gentle Fist style focuses on internal injuries. Each strike, when placed correctly, strikes along the Chakra Pathway System, which is the network of vessels that carry chakra. This network is closely intertwined with the functioning of the body's organs, and so, when the keirakukei take damage, the organs suffer as well. There is no defense against the Gentle Fist, since one cannot train one's internal organs to become stronger. The only possible defense is to stop the Gentle Fist's hits from connecting.

Particularly skilled users of the Byakugan, which appear only rarely, such as Neji Hyūga, can take the Gentle Fist style one step further. Along the keirakurei are 361 pinprick-sized nodes called Tenketsu, or chakra points. When struck, the Byakugan user can choose to either seal, or forcibly open these Tenketsu, thus giving the user complete control over their opponent's chakra system. So, with precision, all those points along the victim's body can be sealed, stopping the chakra flow to those areas. Neji Hyūga is the rare exception, having taught himself the ultimate Hyūga technique, Eight Trigrams Sixty-Four Palms, along with Hinata Hyūga, who, while not able to do 64 palms like Neji, is able to do Eight Trigrams Thirty-Two Palms, a slightly weaker version of Neji's. In his battle against Kidōmaru, Neji attacked with Eight Trigrams One Hundred Twenty-Eight Palms, albeit only in the anime. Able to strike without force, and yet do more damage to a ninja than some of the most powerful ninjutsu, the Gentle Fist is the greatest taijutsu style known in all of Konohagakure.

Some Byakugan users like Hinata can activate one eye only. It's unknown if this has anything to do with their performance, or what its uses are. Hinata showed off this difference while attacking Pain trying to protect Naruto, going as far as giving up her own life to try and save his own, while confessing to him the love she had for him.

Trivia

 * Sometimes the users of Byakugan make some hand seals when activating the ability, though it is also able to activate without any seals. The used seals are: Horse, Tiger, Boar with nearly straightened fingers, Hare, Rat, Dog, Horse, Dog, Hare, Tiger with lifting just index fingers, Rat, Boar with nearly straightened fingers, Dog and Snake with the right index finger lifted straightly. Sometimes just the last seal is used, and most commonly no seals are used.