There are 2 forms of this ring: 

The standard regeneration ring restores 1 hit point of damage (and will replace lost limbs or organs eventually also) per turn. 

It will bring its wearer back from death (but if poison is the cause, the saving throw must be made or else the wearer dies again from the poison still in his or her system). Only total destruction of all living tissue by fire or acid or similar means will prevent regeneration. 

Of course the ring must be worn, and its removal stops regeneration processes.

The rare form is the vampiric regeneration ring. This ring bestows one-half of the value of hit points of damage the wearer inflicts upon opponents in hand-to-hand (melee, non-missile, non-spell) combat immediately upon its wearer (fractions dropped). It does not otherwise cause regeneration or restore life, limb or organ. 

To determine which type of ring is discovered roll percentile dice: 

01-90 ring of regeneration

91-00 vampiric regeneration ring

In no case can the wearer’s hit point total exceed that initially generated.