What is Sunni view regarding Imam Mahdi?

With respect to the appearance of the savior, I ought to point out that in accordance with the perspective of Shiite, 12th Imam of Shiite (Imam Mahdi (a.j.)) is going to appear to save the world from the oppression. So my question is that **what is the viewpoint of Sunni regarding this issue?** In truth, do they believe in this matter as well? If so, who is him (according to Sunni)?