From The Heretic Knowledge Vault
A subclass of mage specializing in healing magic. It's not clear whether this class is distinct from the Lifemage subclass or whether the terms are used interchangeably for the same kind of spellcasters. The term seems to be preferred by cultures less magically advanced than Tsuiraku (where the term "Lifemage" is used) -- which is to say, everybody else.
It is implied that Healers can take care of stuff like day-to-day owies; innate difficulties of a purely physical nature like nearsightedness; and even major trauma, like getting one's hand cut off by a Lancea. However, even the best healer can't deal with an instantly fatal head wound. More significantly, they can't deal with the kinds of problems that make some half elves into Errants, which is a fairly major plot point, to put it mildly.
Not everyone capable of casting healing magic is a Healer by specialization. Ian, to name one, would never think of himself as a Healer, even though he's tried to learn some healing magic to help his sister Evelyn. (Evelyn being an Errant, it didn't work, of course.) Sarine routinely uses healing magic, since she tends to find herself in situations where somebody (frequently herself) has been hacked up, and other elves (e.g. Korene) can do it too. Even the monks of the Ensigerum apparently can use healing magic, as when the late Warrel tries to patch up a bullet wound (his own). None of the above exactly qualify as Healers. We don't know what distinguishes Healers from these other magic users who dabble in healing, never having seen an actual Healer at work. Interestingly, Sarine does describe herself as a "reasonably decent healer" when making her contact with Melrin at the Well Traveled Woman.