One ring to rule them all,
One ring to find them,
One ring to bring them all
 and in the darkness bind them
 
The One Ring was forged by the Dark Lord Sauron during the Second Age to gain dominion over the free peoples of Middle-earth.
He then forged the One Ring in the fires of Mount Doom.
Sauron 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 powerful on their own, Sauron was obliged to place much of his own power into the One to achieve his purpose.
With the Ring, he could control the power of all the other Rings, and thus he was significantly more powerful after its creation than before; but by binding his power within the Ring, Sauron became dependent on it.