some indepth information for you
Sharingan is the blood inheritance limit of the Uchiha Clan. The Uchiha are said to be descendants of the Hyuuga, with the Byakugan changing into the Sharingan over time. This doujutsu (eye skill) gives one the ability to read Genjutsu, Taijutsu and Ninjutsu and then defeat it. As such, the clan member can view a technique being performed and almost instantly memorize the movements required to be able to complete it, including the needed chakra moldings and seals used. However the clan member cannot copy techniques that are unique to another bloodline and the clan member must have a body capable of meeting the chakra and physical requirements to do the jutsu. This rarely stops the clan member though, as such stressful situations often push their limits farther and allow them to harness their full potential.
The Uchiha clan member is not granted the Sharingan at birth; it is only activated later in life, typically during a time of emotional stress in the heat of battle. The Sharingan itself can be transferred to another non-Uchiha by a medical ninja, such as happened with Leaf ninja Uchiha Obito to his teammate Hatake Kakashi. Because his body was not born to carry the Sharingan, the eye is always activated and it burns Kakashi's chakra when used. To lessen the impact of the Sharingan on Kakashi's body, he normally keeps the eye hidden and out of use. Also of note, is the fact Kakashi, a non-Uchiha, was able to advance the Sharingan from two tomoe to three tomoe after gaining the eye.
The Sharingan itself contains three tomoe (comma). A higher mastery of the Sharingan can be seen in the creation of a new tomoe in the eye, with three tomoe being the typical highest form of the eye. The rare Mangekyou form is a unique evolution of the Sharingan beyond the three tomoe form, please see its entry for more information. An eye possessing two tomoe can allow the Uchiha clan member to track the movement of an opponent and do some minor copying of an opponents attack. For avoiding incoming attacks, the Uchiha's speed and agility will greatly control whether they can move to avoid it. However, three tomoe will greatly help in this area.
Two tomoe will also allow the ninja to pierce Genjutsu and see the surrounding reality for what it really is. It can also allow the member to pierce another’s body and see their chakra. While the eye is not refined enough to see the tenketsu and chakra circulatory system like the Byakugan, it can view the movement and flow of chakra. This can help them tell if a person is trapped in a Genjutsu, or if an attacker is a real threat or merely a bunshin.
An eye containing three tomoe will allow the ninja to see the image of an attacker’s next move from the slightest muscle tension in their body. This prevents wasteful movement and allows the Uchiha clan member to conserve much needed chakra in battle. It allows them to synchronize their movement to strike at an area moments before their opponent is even in that spot, essentially allowing them to strike out and hit their opponent before they're even there.
Three tomoe also grants the Uchiha the ability to do Genjutsu and hypnosis after locking eyes with an enemy. This can be used to cause their opponent to do an attack of the Uchiha's choosing. This hypnosis can also be used as a Genjutsu counter, which is the ability to turn a Genjutsu spell back upon the original caster without the requirement of forming any handseals.
The Hyuuga clan were one of the founding noble families of Leaf village. The clan itself is said to be the ancestors of the Uchiha clan who possess the bloodline of the Sharingan eye. Each clan member possess the "Byakugan" or "White Eye." When the "Byakugan" is activated, the clan member gains an extra-sensory perception, which gives them penetrating sight and telescopic vision. This allows them to see the internal chakra circulation system in other beings. This vision also grants the member a near 360? view of their surroundings. The higher the skill of the clan member, the further the distance from which they can sense incoming attacks. Unfortunately the vision does have a weakness, a small blindspot that extends outwards from their upper back.
The clan uses this ability in conjunction with their own chakra creating the Juuken (gentle fist) fighting style. By focusing the chakra to their finger tips, they can press the tenketsu or chakra pressure points of their opponents body. When the finger nears the tenketsu the chakra is forced out, collasping the targetted tenketsu for a period of time. The tenketsu serve as the connectors and exit points for the chakra pathways. So when they are stopped by the Hyuuga clan member, the ninja greatly loses his or her ability to utilize their chakra for jutsu.
These great powers lead to internal strife within the family. Because the eye grants such a unique and powerful ability, the clan became possessive and over-protective of it. Seeing only the main family as worthy users and inheritors of the eye, the heads of the clan began to place restrictions on those branch family members not in power. What resulted were two Hyuuga lines, the powerful main family who would represent the line, and the branch family who were directed to protect the main family and bear the cursed seal.
The cursed seal is placed on the foreheads of branch family members. It serves as a means to protect the secrets of the Hyuuga Byakugan eye. If the branch family member was to ever betray the main family or be killed in battle, the seal would be activated to destroy the body's secrets. Thus forcing the branch family to further bow to the will of the main family.