Who is Imam Al Mahdi?

"In summary, we can say that there are a lot of hadith about the Mahdi to be found in the Sunnah of varying quality, running the gamut between fabricated, weak, and good. Hadith graded as authentic are few. However, belief in the appearance of the Mahdi is an established matter, taking all of the evidence together. The Mahdi in Sunni and Shi`ah traditions: Ahl al-Sunnah believe that a man from the household of the Prophet (peace be upon him) will appear at the end of time in a very natural manner, born like any other person is born. He will live just as others live. He may even fall into error and need people to correct him just like anybody else. Then Allah will decree a lot of good for the Muslims to come at his hands, as well as piety, justice, and virtue. Allah will unite the Muslims around him. There is nothing more to it than this. This is what is found in the hadith.
There is no textual evidence telling us that it is an act of piety to wait for him or to anticipate his coming. It is not appropriate for any Muslim to accept such a claim on the basis of mere supposition. A claim must be backed up by sufficient evidence. Those making claims are many. This has been the case since the dawn of history, as I will soon make clear. A Muslim is expected to examine matters, verify them, and approach them with caution. A Muslim must never be hasty and accept matters on the basis of personal desires or wishful thinking.
Likewise, no aspect of Islamic Law is dependent on the appearance of the Mahdi. Claims that such Islamic institutions as the Friday prayer, congregational prayers, jihad, carrying out the prescribed punishments, or applying the laws of Islam are contingent on his appearance are baseless. Muslims must conduct their lives as normal. They must perform their acts of worship, fulfill their duties, engage in jihad, reform their societies, learn their religion, and teach each other. When this pious man does appear and his identity is established with unambiguous, indisputable proofs, then we should follow him. This was the attitude of the Companions and those that followed them. The scholars throughout the ages have adhered to this view. The idea that the Mahdi should be awaited and anticipated and the excessive emphasis placed on him was a much later development. "
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