The Rosetta Stone is a stone with writing on it in two languages (Egyptian and Greek), using three scripts (hieroglyphic, demotic and Greek) and was originally carved in 196 BC, during the reign of Pharaoh Ptolemy V. The stone is typical of the period, in which complimentary inscriptions praising the Pharaoh for his virtues were common.
Because Egypt had recently been conquered by the Greeks, the Rosetta Stone's inscription was written in both Egyptian and Greek. The Egyptian portion was written in both demotic -- a type of common, everyday alphabet -- and the more ancient and formal hieroglyphs.
The Ptolemic pharaohs were all descendants of General Lagus, friend and ally of conqueror Alexander the Great. Therefore, they were ethnically Greek rather than Egyptian. At the time of the Rosetta Stone's inscription, both Greek and Egyptian languages were used in Egypt. Interestingly, Alexander the Great is buried in the Egyptian city named for him, Alexandria.
When Egypt became a vassal state of the Roman Empire, ruled by a Roman governor, hieroglyphs fell into disuse, and by the fourth century AD the writing system was lost. At the time that the Rosetta Stone was rediscovered, hieroglyphs were indecipherable. Some people even thought they weren't a language at all, but some iconic form of decoration. With the Rosetta Stone, it was possible to decipher the demotic from the Greek, and then the hieroglyphs from the demotic.
The Rosetta Stone was translated and deciphered by Jean François Champollion, a French linguist of amazing ability. He began work on the translation in 1808, when he was eighteen, and in 1822 published his translation of the Rosetta Stone. His breakthrough was in realizing that the images that made up hieroglyphs were used not only as pictures to represent ideas, but also as letters that represented specific sounds, much like American Sign Language.
Thus words that did not have a pictorial representation in the alphabet could be spelled out. Proper names of individuals were spelled out in this way, and names were differentiated from other text by being enclosed in a rectangle with rounded corners, called a cartouche. The Rosetta Stone has led to the translation of hieroglyphs and all the inscriptions in the myriad tombs and monuments of ancient Egypt.