From the British Museum web site:
"Although accounts of the Stone's discovery in July 1799 are now rather vague, the story most generally accepted is that it was found by accident by soldiers in Napoleon's army. They discovered the Rosetta Stone on 15 July 1799 while digging the foundations of an addition to a fort near the town of Rashid(Opens in new window) (Rosetta) in the Nile Delta... The writing on the Stone is an official message, called a decree, about the king...
"The important thing for us is that the decree is inscribed three times, in hieroglyphs, Demotic (the cursive Egyptian script used for daily purposes, meaning 'language of the people'), and Ancient Greek (the language of the administration... The importance of this to Egyptology is immense. When it was discovered, nobody knew how to read ancient Egyptian hieroglyphs. Because the inscriptions say the same thing in three different scripts, and scholars could still read Ancient Greek, the Rosetta Stone became a valuable key to deciphering the hieroglyphs."