"As long as you're not dead, you'll be as good as new."[1]
Heal is a Holypriest healing spell. It is the largest single-target direct heal available to the class, and the spell of choice if your target needs a large amount of healing, but isn't in any immediate danger. It is the slower but more affordable heal, as opposed to the faster but less cost effective [Flash Heal].
Prior to patch 6.0.2, this spell was known as [Greater Heal]. The former [Heal] was a significantly weaker and slower spell, which was removed in the same patch.