Who are the two witnesses in the book of Revelation? I have read tidbits about the two witnesses in the book of revelation. What I don't know is whether or not these are Angels, or Humans and what their purpose is.
I believe that they are literal men. With that in mind, there are two different schools of thought that I have come across on whom these men could be:


These are just two "normal" men who have been called out (like the 144,000 witnesses in Rev. 7:1-9) to perform signs to the unbelievers during the Tribulation.
These are two men that have lived before on earth, and are sent back by God for this special mission. The three most often mentioned are:


Elijah - A great and powerful prophet from the Kingdom period of Israel. He called down fire from Heaven, as do these two witnesses (cf. 2 Kings 1:10, Rev. 11:5). There was a great drought during part of his ministry, similar to the two witnesses in Revelation (cf. 1 Kings 17:1, Rev.11:6). He also appeared on the Mount of Transfiguration with Moses (Mat. 17:1-9), after which Christ stated that he would come back to the earth before the end (Mat.17:11). Last, he was taken up to heaven without dying (2 Kings 2:11); some believe this was so he could come back to earth in the last days.
Moses - Also a great prophet who did many miracles. Against the Egyptians he caused water to turn to blood, along with many other plagues, similar to the two witnesses in Revelation (cf. Ex. 7:20, Rev.11:6). Also, apparently God sent Michael the Archangel to take Moses' body after he died, possibly to preserve it for this future mission (Jude 9). He also appeared on the Mount of Transfiguration with Elijah.
Enoch - A mysterious prophet that we know very little about. Enoch, like Elijah, never died (Ge. 5:24), this being taken by some to indicate that he and Elijah will someday return.



Summary: We don't know who they are, but they could be two future prophets, or two prophets from the past God has sent back.