Itachi is Sasuke‘s older brother and these two are the only ones left in the Uchiha clan. The entire clan was murdered by Itachi about four years ago. Itachi seemed to hold a grudge against his clan and the only reason he gave for his crime was he wanted to test his strength.
He succeeded in killing everyone except Sasuke. He left his little brother alive, telling him “Foolish little brother… you can’t kill me, your hate’s not strong enough… you’re not strong enough. That’s right, live in shame, run away, and live.” Sasuke has done this, living only to avenge his family and become strong enough to kill Itachi. However, killing Itachi is not going to be easy. Itachi at age seven was already a ninja, at eight had mastered the Sharingan (a special ability of the Uchiha), and at 13 had destroyed an entire tribe. He was able to stand up to Kurenai, Asuma, and Kakashi (all of whom are the highest ranking ninjas, and Kakashi being Sasuke‘s sensei). He was also able to defeat Orochimaru who is said to be one of the most powerful ninjas of Leaf Village.
Once he killed his family, he fled from the Leaf Village and joined a terrorist organisation called Akatsuki and quickly became a top member. He has returned to Leaf Village on a mission to kidnap Naruto so they can use the demon inside him. His strongest attack is the Mangekyou Sharingan which is the ability to enter another’s mind and make them feel anything you want. For example, when fighting Kakashi, Itachi forces Kakashi to feel as if he is being stabbed with katanas for 72 hours in only a matter of seconds. This leaves Kakashi unconcious for several days.
Itachi’s ninja level is unknown, though he was described by Asuma as an S-ranked criminal. He graduated top of his Academy class at age seven, had mastered the Sharingan by age eight, became a Chuunin at ten, and was previously an ANBU squad leader at the age of only 13. Now 17, he is feared even by Orochimaru. He left the Leaf Village some years ago, after destroying all but Sasuke of the Uchiha clan.
After the death of the Third Hokage, he returns to the village and encounters Kurenai, Asuma, and Kakashi with his comrade, Hoshigaki Kisame. With no intention of letting them leave quietly, Asuma, Kurenai, and Kakashi fight them, but are very easily defeated.
F: Basically, loosers who can only do stuff academy ninjas can, or aren’t ninjas and have very low skills as a warrior.
D: Basically, a Genin or someone who has done a large amount of misdemeanors.
C: Chunin or someone who has done a lot of misdemeanors AND a few bigger stuff such as a murder and stuff.
B: Chunin or Jounin level, or someone who has done quite a bit or criminal activity such as murders and assassinations.
A: This is held in reserve for people of at least Jounin level and does major crimes without discrimination.
S: This is held in reserve to that of a major threat like someone capable of equaling a Kage and has done severe criminal activity like a lot of assassinations of major people and other TERRORIST-like activities.
U: This basically translates to a super S-Class, and relates to anyone who is at a level were they can take on multiple S-Class ninjas with ease.
Now this type of ranking system is also used in real life by military intel bureau and international crime management organizations. In both the manga and anime, many people, including Jiraiya in episode 81 and 82, feel that all of the members in Akatsuki are in fact “s-class”. So, in conclusion, lets just say he is a major bad ass.