Revelation 13:11-18 has caused much speculation over the years.
11 Then I saw another beast rising out of the earth. It had two horns like a lamb and it spoke like a dragon. 12 It exercises all the authority of the first beast in its presence, and makes the earth and its inhabitants worship the first beast, whose mortal wound was healed. 13 It performs great signs, even making fire come down from heaven to earth in front of people, 14 and by the signs that it is allowed to work in the presence of the beast it deceives those who dwell on earth, telling them to make an image for the beast that was wounded by the sword and yet lived. 15 And it was allowed to give breath to the image of the beast, so that the image of the beast might even speak and might cause those who would not worship the image of the beast to be slain. 16 Also it causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead, 17 so that no one can buy or sell unless he has the mark, that is, the name of the beast or the number of its name. 18 This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666.
I think William Hendriksen gives by far the best explanation in his outstanding commentary on the book of Revelation, More Than Conquerors.