Orochimaru one of the Legendary Sennins of the Leaf Village, until his thirst for the knowledge of Forbidden Techniques exiled him from the village. Since then, he has moved on from body to body in a technique that transfers his soul into another one in a bid for immortality. His next victim, or prodigy, as he states, is Uchiha Sasuke.
After his initial meeting with Sasuke in the Death Forest, Orochimaru gives him a curse seal as tantalizing bait; using Sasuke's ambition against himself, so that Sasuke would one day follow him and Orochimaru would claim his greatest power. The Sharingan.
He also was once part of the Akatsuki, the same organization that Uchiha Itachi and Hoshigaki Kisame are currently in, but left because he feared Itachi to a great magnitude.
Orochimaru has terrifying offensive techniques, and the exact opposite of Jiraiya. His ability to make Kakashi freeze dead in his tracks by just showing off his killing intent should be indicator of how strong he is.
Don't let him bite you.