The Mark of the Beast is a symbol of loyalty to the Beast out of the Sea of the Book of Revelation. Everyone is to take this Mark or be cut off from the society. Christians must not take this Mark because it is not of God but of the Beast.
The Beast out of the Earth created an image and made it alive. The image ordered everyone to recieve a Mark on their right hand or their forehead as a symbol of their loyalty to the first Beast; anyone who didn't couldn't be a part of the society. They couldn't buy or sell without the Mark. What the Mark is is unknown; what is known is its meaning: a symbol of loyalty to the first Beast. Some claim it is a microchip (because of the King James Version's use of "in" in Revelation 13:16), or even a tattoo. If it is required to buy or sell and is a symbol of loyalty to the first Beast (whatever it may be) then that is the Mark of the Beast; everything else is not the Mark.
Microchips themselves are not the Mark of the Beast nor a precursor, although a specific one may be required. Tattoos themselves are not the Mark of the Beast, although a specific one may be required. Not everyone has to have the Mark of the Beast on/in them. Another possibility is to have the the name of the Beast or the number of its name on you. Its number is 666 but its name is currently unknown to us. However many have tried to determine what the name is; some have figured out Nero, "The Great Beast", or even Barney the dinosaur. They determine this by isopsephy, which places a numerical value to letters and adds them together. The sum of these names in some form is 666. There likely are millions of possible names that add up to 666, but there is one that is the name of the Beast.
Christians are not to have this Mark. Since the Mark is a symbol of a person's loyalty and devotion the first Beast, a Christian who has the Mark is essentially denouncing Christ and living a life of idolatry — worshiping the Beast. Those who have victory over the Beast and its Mark by refusing to have it will in Heaven standing on the Sea of Glass that is around God's throne. The Christians who do not take the Mark but were beheaded will live and reign with Christ for 1000 years during His reign on earth.
Those who have the Mark will undergo God's wrath and be tormented with fire and sulfur. One of the bowl judgements will be harmful and painful ulcers on those who have the Mark. Another one of the bowl judgements will cause darkness to fill the Beast's kingdom and the Beast's subjects who have the Mark to gnaw their tongues in pain. They will blaspheme God because of these two judgements.