The Uchiha-clan is the most famous and most powerful clan of the Leaf. They have the rare and very famous Sharingan advanced bloodline ability. They are know throughout the world for their power. At least, the Uchiha-clan would be the most powerful if it still existed. A terrible tragedy has overcome this clan. Uchiha Itachi, Sasuke's older brother, single handedly wiped out the entire clan. He left only one survivor: Sasuke.
After the massacre Itachi left the Leaf and continued his quest for power. Sasuke on the other hand, was left behind alone and confused. Now he lives only to take revenge on his brother and try to revive the Uchiha-clan once more.
In Itachi more then Sasuke, you can clearly see the pure power and genius that flows thought the blood of all members of the Uchiha-clan. Not to mention the fact the that they have Sharingan (the most wanted advanced bloodline ability of all!), they truly are natural born ninjas/killers. At the very young age of 10, Itachi became a chuunin. And now he might even be the most powerful ninja alive. Although Sasuke isn't quite as impressive as his older brother, he still is one of the most talented rookies around.
The sharingan is a special eye ability that is only apparent in the Uchiha clan. It is an advanced characteristic in the pupil of the person's eyes. It is said that the sharingan evolved from the Byakugan, which is excluded from any possible use by anyone except for members of the Hyuuga clan. In terms of mental, emotional, and physical insight, the Byakugan is far more superior than it's succesor. The sharingan has three semi-colons. It fits well since the sharingan can see through and mimic three types of jutsu, which are ninjutsu, genjutsu, and taijutsu.
The sharingan's main ability is the mimicery of other jutus. It will copy any ninjutsu, genjutsu, and taijutsu. Of course it can also copy the movements of the opponent by studying him or her carefully. The more times the person uses the sharingan, the more jutsus they copy. If they continue to keep the sharingan out, they will continue to copy jutsus even if it's against their will. It acts automatically like a car alarm. If it sees a jutsu, it starts to act up and acts like a defensive system. Like a car's defensive system is an alarm, the sharingan's defensive system is to copy what they see. Although, the sharingan's accuracy of mimicing the techiniques is high, it doesn't gurantee complete mastery of the jutsu. For incredibly difficult and extremely high-level jutsu, it requires practice. Of course the sharingan can speed up the process. It can first copy what hand seals are required (if any at all) and then all they have to do is find the right amount of chakra to exert. If it's a move that requires no hand seals then all they have to do is exert the right amount of chakra into the right place. If it's a taijutsu technique then they will have no problem with that. They can just remember what movements they copied and practice on that. Another ability is that it can see through illusions. For example if there were many bunshin or Kage bunshin all the sharingan user would see is balls of chakra that look like flames. This allows them to seperate what is real and what isn't.
Another 'ability' of the sharingan is the ability to supposedly look into the future. In other words, they can trick they're opponents into thinking that they are fighting a seer. It's just an illusion. A series of different steps are used to create this illusion. First the user makes it seem that they can read minds by using a hypnotic genjutsu and copying their opponents thoughts. For all these steps to work the user and the opponent must make eye-contact. Same goes for the movement copying. The user of the sharingan stares their opponent in the eyes while copying all their movements. The most important thing for a person that does this to do is to make eye-contact.
The sharingan uses up a lot of chakra. However, this only applies if the sharingan user is not an Uchiha. Since they don't have the right blood, they can't use it as effectively and they get exhausted more easiy then members of the Uchiha clan. For example, Kakashi gets exhausted quickly everytime he uses his sharingan. However, Neither Itachi nor Sasuke feel the same way. They are both members of the Uchiha clan so they have a direct heritage of the sharingan. Pretty much, blood plays a big role in the chakra usage of the sharingan. If you have the right blood, you don't get tired easily. If you don't have the right blood then be more careful when you use the sharingan.
The sharingan also many disadvantages such as speed. It can't see speed very well because most of the time it's not chakra based speed. It's harder for users of the sharingan to see speed. That's what makes taijutsu a dangerous enemy to it. Combining taijutsu and speed will give an advantage over the sharingan. Therefore, the sharingan user will have lots of trouble seeing their enemies come at them. Another weakness of the sharingan is that it can't copy bloodline based jutsus. It's just not theirs to copy. Basically, physical combat is the most dangerous to the sharingan. It's not only unpredictable but almost unstoppable for sharingan users. However, it's wickedly powerful in the areas of copying ninjutsu and genjutsu.
Mangekyou Sharingan, Tsukuyomi. Uchiha's ultimate genjutsu. So far it's only been used by Itachi. The focus of this genjutsu is to cause a breakdown in someone's mind and spirit. First Itachi looks at his opponent in the eyes. He then chooses a form of torture or a past event that hurt that person the most. Also Itachi can make the person feel a certain amount of time in an instant. For example, Itachi uses a form of torture on Kakashi. Itachi made Kakashi think that he was really getting katanas shoved into himself. Itachi, being the master of the illision was doing all the shoving. Itachi also made it seem to him that it felt like 72 hours (three days). In a form of a past memory, Itachi made Sasuke relive the day their parents were killed by Itachi. This only lasted for 24 hours. This genjutsu causes a lot of pain that lingers around for a while. However, it's also apparently dangerous to Itachi as well. He can't seem to use it much because it could be fatal