Just quoting Wikipedia:
In the Textus Receptus manuscripts of the New Testament, the Book of Revelation (13:17–18) cryptically asserts 666 to be "the number of a man," associated with the beast, an antagonistic creature that appears briefly about two-thirds into the apocalyptic vision. Some manuscripts of the original Greek use the symbols ??? chi xi stigma (or ? digamma), while other manuscripts spell out the number in words.
In modern popular culture, 666 has become one of the most widely recognized symbols for the Antichrist or, alternatively, the Devil. The number 666 is purportedly used to invoke Satan. Earnest references to the number occur both among apocalypticist Christian groups and in explicitly anti-Christian subcultures. References in contemporary Western art or literature are, more likely than not, intentional references to the Beast symbolism. Such popular references are therefore too numerous to list.
It is common to see the symbolic role of the integer 666 transferred to the digit sequence 6-6-6. Some people take the Satanic associations of 666 so seriously that they actively avoid things related to 666 or the digits 6-6-6. This is known as hexakosioihexekontahexaphobia.
In a couple of early biblical manuscripts, the number is cited as 616.