The One Ring was an ancient artifact created by the Dark Lord Sauron in the Second Age for the purpose of ruling over the Free peoples of Middle-earth, mainly the Elves.
One Ring has a special inscription in the Black Speech of Mordor:
ash nazg thrakatulûk agh burzum-ishi krimpatu
translates to:
One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them.
