The mark of the beast is the sign of the Antichrist, and is mentioned in Revelation 13:15-18:
The second beast was given power to give breath to the image of the first beast, so that the image could speak and cause all who refused to worship the image to be killed. It also forced all people, great and small, rich and poor, free and slave, to receive a mark on their right hands or on their foreheads, so that they could not buy or sell unless they had the mark, which is the name of the beast or the number of its name. http://christianity.about.com/od/endtimestopicalstudy/a/Mark-Of-The-Beast.htm
Some people believe the mark of the beast is the credit card companies plan, to place credit card numbers on the back of the hand of all their customers by the year 2026.
Sometime in the future, the anti christ (satan) will come and claim to be christ. And many will be decieved and think he's really christ and they will go will him. But in truth, Christ comes after anti christ....
This is the antichrist during the 7 year tribulation.
16 He causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads, 17 and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name.
18 Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.
the mark of the beast, it is uncertain however how this mark will manifest itself upon those that have received the mark, is it a digital chip? A vaccine? A tattoo? And what does the 666 mean, besides it being the mark of the beast what is it that makes it the mark? Your question lead to me to more questions.