Heal is one of a priests healing abilites, this comes after Lesser Heal and before Greater Heal in terms of spell name progression. This is usually dumped when a priest earns Flash Heal at level 20.
This section concerns content exclusive to Cataclysm.
In the Cataclysm priest review it was announced that because of the existing version becoming obsolete at higher levels, it is something intended to change in Cataclysm. Introduced at a low level, the "new" Heal spell will functionally work much like a down-ranked Greater Heal that is intended to heal for a moderate amount and for when efficiency is an issue (making Flash Heal the incorrect spell for the job).