Saruman was one of the 5 Istari sent to Middle-Earth to help guide the free peoples of Middle-Earth to victory against Sauron. Saruman usually wore white, and was appointed as head of the White Council. He was named Curunir (the Cunning) by the elves, and his greatest power was his voice, which always sounded like the voice of reason to any weak-minded listeners.
Saruman became entranced by the history of the Rings of Power, and learned from the records of Minas Tirith that the Ring had fallen into the river Anduin, and began searching for it. He sought the power of the One Ring, both by acquiring the One Ring itself and by making a ring of his own. While using the Palantir of Isengard Saruman was detected and ensnared by Sauron, who had another of the 'seeing stones'. Saruman decided to betray his duties and aid Sauron, though his real intention was to take the One Ring for himself and overthrow Sauron with it, yet also to take all the power of the rings for himself.
In the book, the Two Towers, he made his own ring, changed his robes, and called himself 'Saruman of Many Colours'. In the movies his ring is not mentioned, and he remains in white robes.