Neji is a member of the Hyuga clan and possesses the Byakugan. He is a child prodigy and a genius. But as a member of a branch family of the Hyuga clan he is ineligible to learn their most secret techniques. Despite this however, he has managed to deduce their workings simply through observation, and has learned many of them.
Neji is highly proficient with the Gentle Fist style of hand-to-hand combat, which, in combination with the byakugan is used to attack an enemy's Chakra pathway system (the network of vessels which carry chakra), thus causing internal injury.
On the third birthday of Hinata (his cousin) when Neji was four years old, he was branded with the Main family's Juinjutsu which serves two purposes. The first is to seal their byakugan ability when they die (shielding its secrets from enemies) and the second is to control the branch family members with the ability to destroy their brain cells with a secret hand seal.