Who are the two witnesses?
The two witnesses are two Spirit-empowered prophets who appear in Jerusalem during the 1,260 days (three and a half years) of the Antichrist's reign, described in Scripture as the two olive trees and two candlesticks standing before God, and given power reminiscent of Elijah and Moses.
Revelation 11 introduces two witnesses who prophesy for a thousand two hundred and threescore days, clothed in sackcloth. They are called the two olive trees and the two candlesticks standing before the God of the earth, language drawn from Zechariah's vision of two anointed ones who stand by the Lord of the whole earth. This shows they are set apart in a special anointed role during this time.
Their ministry carries power like that of two great Old Testament prophets. Like Elijah, who declared there would be no rain and was obeyed, the two witnesses have power to shut heaven so that it does not rain during the days of their prophecy. Like Moses, who turned the waters of Egypt to blood, they have power over waters to turn them to blood and to smite the earth with plagues as often as they will. Fire even proceeds from their mouths to destroy anyone who tries to hurt them.
They minister for three and a half years, the very same span of time in which the Antichrist brings desolation and abominations upon the nation of Israel. Their preaching happens while the Body of Christ is still on the earth. When they finish their testimony, the beast that comes up from the bottomless pit makes war on them, overcomes them, and kills them, and their bodies lie in the street of Jerusalem, called spiritually Sodom and Egypt, for three and a half days while the world rejoices over their deaths.
After three and a half days, the Spirit of life from God enters them, they stand on their feet, and a voice from heaven calls them up. They ascend to heaven in a cloud while their enemies watch, and immediately a great earthquake strikes, killing seven thousand people and destroying a tenth of the city, causing the survivors to fear and give glory to God.
The scriptures
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. And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them. And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified. And they of the people and kindreds and tongues and nations shall see their dead bodies three days and an half, and shall not suffer their dead bodies to be put in graves. And they that dwell upon the earth shall rejoice over them, and make merry, and shall send gifts one to another; because these two prophets tormented them that dwelt on the earth. And after three days and an half the Spirit of life from God entered into them, and they stood upon their feet; and great fear fell upon them which saw them. And they heard a great voice from heaven saying unto them, Come up hither. And they ascended up to heaven in a cloud; and their enemies beheld them. And the same hour was there a great earthquake, and the tenth part of the city fell, and in the earthquake were slain of men seven thousand: and the remnant were affrighted, and gave glory to the God of heaven.
Then answered I, and said unto him, What are these two olive trees upon the right side of the candlestick and upon the left side thereof? And I answered again, and said unto him, What be these two olive branches which through the two golden pipes empty the golden oil out of themselves? And he answered me and said, Knowest thou not what these be? And I said, No, my lord. Then said he, These are the two anointed ones, that stand by the LORD of the whole earth.
And two olive trees by it, one upon the right side of the bowl, and the other upon the left side thereof.
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.
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.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.