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.
I believe these are literally Moses and Elijah, by comparing what Revelation says about the two witnesses with the Old Testament accounts of Moses and Elijah.
Revelation 11:3-5 And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth. These are the two olive trees, and the two candlesticks standing before the God of the earth. And if any man will hurt them, fire proceedeth out of their mouth, and devoureth their enemies: and if any man will hurt them, he must in this manner be killed. These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will.
First, "fire proceedeth out of their mouth, and devoureth their enemies: and if any man will hurt them, he must in this manner be killed".
The only person in the Bible with this unique characteristic, of calling down fire upon their enemies, was Elijah.
II King 1:9-12 Then the king sent unto him a captain of fifty with his fifty. And he went up to him: and, behold, he sat on the top of an hill. And he spake unto him, Thou man of God, the king hath said, Come down. And Elijah answered and said to the captain of fifty, If I be a man of God, then let fire come down from heaven, and consume thee and thy fifty. And there came down fire from heaven, and consumed him and his fifty. Again also he sent unto him another captain of fifty with his fifty. And he answered and said unto him, O man of God, thus hath the king said, Come down quickly. And Elijah answered and said unto them, If I be a man of God, let fire come down from heaven, and consume thee and thy fifty. And the fire of God came down from heaven, and consumed him and his fifty.
Next, "These have power to shut heaven, that it rain not in the days of their prophecy"
Elijah also caused rain to cease when he was on the earth.
I Kings 17:1 And Elijah the Tishbite, who was of the inhabitants of Gilead, said unto Ahab, As the LORD God of Israel liveth, before whom I stand, there shall not be dew nor rain these years, but according to my word.
I Kings 18:1 And it came to pass after many days, that the word of the LORD came to Elijah in the third year, saying, Go, shew thyself unto Ahab; and I will send rain upon the earth.
I Kings 18:41 And Elijah said unto Ahab, Get thee up, eat and drink; for there is a sound of abundance of rain.
Finally, "and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will."
Who does this sound like? Turning the water into blood? Smiting the earth with plagues?
Exodus 7:19-20 And the LORD spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water, that they may become blood; and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone. And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood.
The story of the plagues in Egypt would take up a lot of space, but most people are already familiar enough with that story.
Elijah and Moses were taken from the earth
Per 2 Kings 2:11, Elijah went to heaven in a fiery chariot and did not see death. Jude verse 9 tells of a conflict between Michael and Satan over the body of Moses (also see the apocryphal Book of Enoch), indicating that his body was removed from the earth.
Both Elijah and Moses appeared at the Mount Transfiguration scene
Luke 9:28-31 And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray. And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering. And, behold, there talked with him two men, which were Moses and Elias: Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.