Life Leech causes damage to the enemy health and restores equal amount of health for player character. Life Leech is not considered an Elemental Damage. Unlike Poison Damage or Bleeding Damage, Life Leech increase by 2.5% per player level ( in Titan Quest: Anniversary Edition ), and not tied to any Attribute.
[X Life Leech over Ys] means that any damage you deal will heal you by X over Y in a similar way as a Potion. It is not stack with itself.
[Life Leech +X%] is increase a potency of Life Leech you inflict, but it don't grant Life Leech by himself.