What is the Antikythera Mechanism?

The antikythera mechanism is currently housed in the Greek National Archaeological Museum in Athens and is thought to be one of the most complicated antiques in existence. At the beginning of the 20th century, divers off the island of Antikythera came across this clocklike mechanism, which is thought to be at least 2,000 years old, in the wreckage of a cargo ship. The device was very thin and made of bronze. It was mounted in a wooden frame and had more than 2,000 characters inscribed all over it. Though nearly 95 percent of these have been deciphered by experts, there as not been a publication of the full text of the inscription.
What is the Antikythera Mechanism?