The Sannin, otherwise known as the Legendary Three Ninja, were the students of the Third Hokage, Hiruzen Sarutobi. They are Jiraiya, Tsunade, and Orochimaru. All three are incredibly talented shinobi who distinguished themselves in many ways during the Second Shinobi World War. They received their name from Hanzō, the leader of Amegakure, an extremely powerful shinobi said to be an icon throughout the shinobi world. Though he bested them in battle, he was impressed by their performance, and titled them such while sparing their lives, believing that the war would eventually end, as it did, in Konoha's victory.
Tsunade - the Slug Princess, is known as the strongest and most beautiful woman in the world and the greatest medical-nin, whose nickname comes from her slug summonings. After a two-decade period of retirement, she returned the Konoha to become the Fifth Hokage following her teacher's death.
Orochimaru - the White Snake, eventually defected from the village when he was caught performing experiments on his fellow shinobi and even children. He was turned down for the position of Hokage because Hiruzen was distrustful of his intentions. His snake summonings earned him his nickname.