Saruman is the wizard called Saruman the White before Gandalf takes that title. He is the head of the Istari, the order of wizards sent to Middle-earth to oppose the rising power of Sauron.
Saruman initially appears as a powerful and respected leader. He resides in the tower of Orthanc at Isengard and studies the history and making of the Rings of Power. His title, Saruman the White, signifies his position at the head of the wizard order. Gandalf is originally known as Gandalf the Grey.
Saruman's desire for knowledge and power eventually corrupts him. He begins seeking the One Ring for himself and allies with Sauron while secretly hoping to replace him. After Gandalf defeats him and returns transformed, Gandalf becomes Gandalf the White, while Saruman is stripped of his authority and later called Saruman of Many Colours.
Radagast, Alatar, and Pallando are other wizards, but none is Saruman. Alatar and Pallando are the two Blue Wizards, while Radagast is associated with animals and the natural world.