The Rosetta stone is an irregularly-shaped piece of black basalt, measuring about 45 inches / 114 centimeters long, and 28 inches / 72 centimeters wide. Broken long ago, the single piece is all that has been found.
The stone's inscriptions, as written by the priests of Memphis, listed contributions of Ptolomy V Epiphanes (see The Ptolemies). It was written in the ninth year of his reign (i.e. about 196 B.C.) to mark his accession to the throne.
The Rosetta Stone's importance to history and archaeology are the consequences of its identical records having been written in two languages, Egyptian and Greek, and three alphabets - hieroglyphic, demotic (a form of Egyptian hieroglyphics) and Greek. With two already known, understanding of the third was then made possible.
The actual painstaking work of deciphering was done by Thomas Young of England and Jean-Francois Champollion of France. It was largely through their efforts that the translation of all ancient Egyptian hieroglyphic writing was made possible.
Fact Finder: In what part of Egypt did the Israelites live before the Exodus?
See The Nile Delta and The Land Of Goshen