Number of the Beast
In the Bible, the Number of the Beast is 666 (or, in some translations, 616), and it is a mark that will be forced upon the citizens of the world by the government of the Antichrist. The penalty for not accepting the number is either death, persecution, or a lack of financial security (Revelation 20:4). Those who do accept it however, will suffer eternal damnation for eternity. According to one verse of the Book of Revelation, chapter 13, verses 16-18:
"And it will cause all, the small and the great, and the rich and the poor, and the free and the bond, to have a mark on their right hand or on their foreheads, and it will bring it about that no one may be able to buy or sell, except him who has the mark, either the name of the beast or the number of its name. Here is wisdom. He who has understanding, let him calculate the number of the beast, for it is the number of a man; and its number is six hundred and sixty-six."
The Number of the Beast is sometimes also referred to as the "Mark of the Beast". It has been commonly used in popular culture, for example in The Omen series of movies. Some people interested in Bible prophecy have pointed to many parallels between the modern economy (such as the use of bar codes and RFID tags) and the potential for a system such as the one described in the book of Revelation.  
Meaning of the Number
Many prophecy teachers regard the meaning of the number to represent the timing of when the identity of the Antichrist is revealed. Since most scholars regard the Abomination of Desolation as the point in time in which the Antichrist is revealed, they postulate that Isaiah 66:6 could denote the Abomination of Desolation, since the verse denotes the commotion within the Temple in Jerusalem. The number 666 refers to the numerical denotion of the verse. The verse reads:
"Hear that uproar from the city, hear that noise from the temple! It is the sound of the LORD repaying his enemies all they deserve."(Isaiah 66:6)
Starting with Isaiah 66:1 and reading through the chapter, one might consider the reason behind the Abomination of Desolation: The Jews are reverting back to the old practice of sacrificing animals- when in actuality, the Mosaic Law was fulfilled in the Atonement of Christ. The Jews have denied Christ and are worshiping in the newly rebuilt temple. In other words, God is using the Antichrist to punish the people for denying his Son, Jesus Christ.
Interestingly, numbers weren't added in the Bible until the invention of the printing press.