The Rosetta Stone is an ancient Egyptian inscribed stone from about 200 BCE.
It was inscribed with a decree issued on behalf of King Ptolemy V. What is important to linguists is that the same decree appears in 3 different languages.
The top is in Ancient Egyptian hieroglyphics; the middle is in Demotic (a form of early Egyptian) and the lower text is in Ancient Greek.
Because the three texts say roughly the same thing they have been used to help decipher and understand the meaning of Egyptian hieroglyphics, hitherto hidden.
Discovery & Translation
In 1799 a French soldier on an expedition in the Nile Delta discovered the stone near the town of Rashid, known as Rosetta in the West. When the British defeated the French in Egypt in 1801 they took the stone and sent it to London where it has been on display at the British Museum ever since (where it is the most visited object there). The Egyptian government have recently begun asking for its return.
Since its discovery many amateur and professional linguists have tried to translate and match the three scripts. They matched names known in Greek to the Demotic and hieroglyphs and using this as a base began to build up a full translation.