Age 54 in Shippuuden.
Orochimaru was one of the 3 Legendary Sannin of the Leaf Village along with Tsunade and Jiraiya, 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 Forest of Death, 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.
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.
He failed at the attempt of destroying the Hidden Leaf Village, and the Third Hokage Sarutobi, before dying, did a special sealing technique Shiki Fuujin, which was able to take Orochimaru's soul, but since he was old and injured he was only able to seal Orochimaru's arms within him.
During the timeskip Orochimaru has been training Sasuke. He taught him various snake-type jutsus as well as helped him improve his Chidori control. Later in Shippuuden, Sasuke decided that Orochimaru is unworthy of having an Uchiha's body. That's when we saw Orochimaru revealing his true form, a large white snake. He tried to take over Sasuke's body, however Sasuke defeats him instead, and absorbs him into his own body. He then gained access to some of Orochimaru's abilities, but had to use a small amount of chakra to keep Orochimaru suppressed.
Next time we see Orochimaru is when the two Uchiha brothers, Sasuke and Itachi, fought. Having used up all of his chakra; Sasuke gets overpowered by Orochimaru inside him, and Orochimaru takes over. However, Itachi was expecting that to happen, and he immediately pierced through him with Susanoo. Orochimaru then realized that he was stabbed by a special weapon, the Totsuka Sword, permanently sealing him and removing the Cursed Seal from Sasuke. Orochimaru, in a form of a little white snake, managed to escape from Itachi's Susanoo, but was later killed by Amaterasu's never-ending flame.