Saruman was the most powerful of all the wizards, and once he was a Maiar (in short, an angel or benevolent spirit) in the realm of Valinor known as Curunir. He was the first to be chosen by the Valar to combat the growing threat of Sauron in Middle Earth, and he chose Alatar the Blue (Morinehetar, chose Pallando to travel with him, and his name was Romestamo) Radagast the Brown (Aiwendil) and Gandalf the Grey (Olorin) as his inital companions.
When they reached Middle Earth by way of the Grey Havens, an elvish port in the northern Elvish realm of Lindon, they were greeted by Cirdan the Shipwright. Cirdan had in his possession one of the three great Elvish rings, Narya, the Ring of Fire (cue Johnny Cash). Instead of giving the powerful ring to Saruman, Cirdan instead withdrew the ring to Gandalf, who he deemed to be more powerful. This started a silent jealousy of Saruman towards Gandalf. 
Later, in the Third Age, the White Council met for the first time since the defeat of Sauron. Saruman was made aware of Thorin's Company, the reawakening of Dol Guldur, and Radagast's involvement in all three, none of which he was particularly approving of; he dissaproved of the courageous Dwarves, denounced the theory of Sauron's return, and admonished Gandalf's trust of the mushroom-eating wizard.
Even later in the Third Age, Saruman resided in the old Gondorian fortress of Orthanc, where he saw much of Middle Earth through his Palantir. As he was expert on all things related to Sauron (the reason for his summoning), he was enticed by the power of the Ring, and resolved to become a servant of Sauron, whom he now knew to be awakened, and planned to later overthrow him and take the Ring for himself. This, however, proved to be his undoing. He was murdered by his corrupted servant Grima Wormtongue on the pinnacle of Orthanc in the extended edition of The Return of the King (his throat was slit by Wormtongue after the Scouring of the Shire in the book's final chapters.).