The One Ring grants the wearer invisibility and cannot be seen by almost all eyes. The exceptions include those that can sense the power when it is being worn (i.e. Nazgul and the Eye of Sauron). The Ring is a source of great power that corrupts man and holds the very essence of Sauron himself. When The Ring was removed, Sauron's physical form vanished, retaining only his essence. Should the Ring be returned to the Eye, Sauron would return to power and destroy the world of men.
The Ring seems to instill greed to those vulnerable to its evil. It can drive a person to do whatever it takes to make the Ring their own (even murder). There are those that have resistance to its temptation.
Men have made attempts to use the Ring for their own in order to use it against Sauron's Army, blind to the fact that it cannot be used except by Sauron.
The Ring seems to have a mind of its own. At times, the Ring may fall off of the wearer, or to become misplaced. The Ring wants to be found and it cannot be destroyed except by being thrown into the Crack of Doom in Mount Doom of Mordor.
There is an inscription in the language of Mordor that is revealed when exposed to fire.
One Ring to rule Them all,
One Ring to find Them,
One Ring to Bring Them all
and in The Darkness, Bind Them