The Heal skill is both an Active and passive skill.
The Heal skill is actively used to save incapacitated players. This skill uses 1 healer's kit per attempt. The incapacitated player can be revived but they will only be healed to 1 hit point if a successful Heal check is made. To make a successful Heal check, your Heal skill + 1d20 must equal or exceed the DC check for reviving an incapacitated player. The DC of the Heal check to revive a wounded player is 20.
The Heal skill is passively used to determine the number of hit points that are regained at rest shrines. The number of hit points regained at a shrine is determined by the highest Heal skill of any nearby ally (thus, it is helpful for Clerics or other high-Wisdom characters to stand in rest shrines when someone else is using them).
From Update 19, the Heal skill is now much more useful, as it adds 1 point of Positive and Negative Spell Power per point of skill.
This enhances Positive Energy spells (such as Cure Wounds or Heal) and Negative Energy spells (such as Inflict Critical Wounds) and Positive/Negative spell-like abilities for all classes. It does not affect spells cast from scrolls or clickies.