The One Ring was created by the Dark Lord Sauron during the Second Age in order to gain dominion over the free peoples of middle-earth. In disguise as Annatar, or "Lord of Gifts", he aided the Elven smiths of Eregion and their leader Celebrimbor in the making of the Rings of Power. He then forged the One Ring himself in the fires of Mount Doom.
He intended it to be the most powerful of all Rings, able to rule and control those who wore the others. Since the other Rings were themselves powerful, Sauron was obliged to place much of his native power into the One to achieve his purpose.
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 the other Rings, and thus he was significantly more powerful after its creation than before; and putting such a great portion of his own power into the Ring ensured Sauron's continued existence so long as the Ring existed. On the other hand, by binding his power within the Ring, Sauron became dependent on it — without it his power was significantly diminished.