The Byakugan is a special pupil condition that naturally occurs in members of the Hyuga clan (and thus only characters from the Hyuga clan possess it). Byakugan users are characterized by their lavender, pupil-less eyes. When the Byakugan is activated, their pupils appear and the veins near their temples bulge.
A person with the Byakugan has a nearly 360 degree field of vision except for a small blind spot behind the first thoracic vertebra (near the back of the neck), which is its only known weakness. Byakugan users can detect anything around them within a 50-meter radius (although Neji Hyuga manages to increase the radius to over 800 meters during the timeskip), making them close-range combat experts. The Byakugan also gives the user the ability to see through basically any matter over extremely long distances. This seems to be a combination of x-ray and thermographic vision, as Hinata notes in the Bikochu filler arc that objects can be difficult to see if their temperature is the same as the objects around them. A small degree of telescopic vision is also present, as a Byakugan user can focus on a single object at any point within their range of vision. The degree of focus seems to be dependent on the skill of the user. The Byakugan's sight cannot be blocked, except in the case of something like Ranmaru's kekkei genkai ability, unlike the Sharingan, which can be blocked by any sufficiently non-transparent substance. It is however, possible to impede it slightly through the use of certain powerful seals. The Byakugan's insight capacity makes it useful for searching, tracking, and identifying targets.
One of the most powerful aspects of the Byakugan is the ability to see the inner chakra coil system, which is the key aspect of using Gentle Fist to attack said system. If sufficiently developed, the Byakugan is able to see individual chakra points, potentially allowing the user to shut off the opponent's chakra flow completely, such that the opponent cannot perform techniques that require chakra. Seasoned Byakugan users are even capable of killing with a single blow.
Protection of the Byakugan ability is very important to the Hyuga clan, and has led to the development of an elaborate family hierarchy. The Hyuga clan is divided into two houses: the main house and the branch house. The branch house's main purpose is to protect the main house and the secrets of the Byakugan. Branch house members' abilities and freedom are restricted through the means of a cursed seal that is placed on every branch house member's forehead. Through the means of a special technique that only main house members know, the seal can be activated, destroying the brain cells of the person branded with the seal. This allows the main house to make the branch house subordinate to its wishes. In addition, whenever a branch house member dies, the cursed seal will also seal the Byakugan ability so that enemies will not be able to discover the secrets of the bloodline limit if Konoha's hunter-nin are unable to find and dispose the body before the enemy finds it.
There are several techniques developed specifically for Byakugan users, which complement and make use of the Byakugan's abilities – most notable among which are the defensive barrier technique Eight Trigram Palms Heavenly Spin and the high-speed precision attack Eight Trigrams Sixty-Four Palms.