Who is Istari from The Silmarillion and what is their importance?

The Istari, or wizards, appear in the final chapter as a gift from the Valar to combat Sauron. They are Curunir, Olurin, Radagast and Saruman. Men refer to Curunir as Saruman, meaning Man of Skill, and Olurin as Gandalf, though the Elves refer to Olurin as Mithrandir or the Grey Pilgrim. Olurin is a Maia as well as being an Istari. Curunir means one cunning devices.