The One Ring was also known as Ruling Ring, the Master Ring, the Doom of Man, the One Ring, the Ring of Power, and Isildur's Bane.
Sauron disguised himself as Annatar, or "Lord of Gifts", and aided the Elven smiths of Eregion and their Celebrimbor in the making of the Rings of Power. He then forged the One Ring himself in the fires of Mount Doom. But creating the ring simultaneously strengthened and weakened Sauron's power. On the one hand, as long as Sauron had the ring, he could control the power of all other Rings of Power, and thus he was significantly more powerful. On the other hand, by binding his power within the Ring, Sauron became dependent on it. Without it his power was significantly diminished.
The words of the Ring-inscription are in Black Speech, a language devised by Sauron and used in the land of Mordor. The inscription reflects the One Ring's power to control the other Rings of Power. The writing uses Elvish letters, in a mode adapted to the Black Speech.
Normally the One Ring appeared perfectly plain and featureless, but when heated its inscription appeared in fiery letters. A drawing of the inscription and a translation provided by Gandalf appears in Book I, Chapter 2 of The Fellowship of the Ring, "The Shadow of the Past".
Gandalf speaks the words in Black Speech in Book II, Chapter 2, "The Council of Elrond":
Ash nazg durbatulûk, ash nazg gimbatul,
Ash nazg thrakatulûk agh burzum-ishi krimpatul.
Translated, the words mean:
One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them.