Healers unsurprisingly specialize in healing magic. While the concept is straightforward, there are a few nuances to their art that keep them in check. Healers are unable to resurrect the dead or re-grow limbs or organs. They may be able to reconnect an arm, providing the person can locate the arm quickly enough, but something like decapitation is non-reversible as the person dies instantly. They are able to cure most illnesses.
Obviously Healers are essential to any group hoping to succeed in battle. They are the only class who are able to heal very major wounds. (White Mages are limited in ability and Time Mages are... complicated.) They're pretty useless when forced into an offensive role, but a Healer who has another class under their belt can be a formidable opponent. They naturally pick up other magical classes, so it is common for Healers to double as Geomancers or the like.
Healing is a great ability to have in the everyday sense. It's a very profitable business to get into. Because a strong Healer is required for the resurrection process, many Healers are rich - especially if they offer their services to the masses on the side. People love Healers. The village Healer is revered and can expect to be fixing a lot of children who've gotten nasty scrapes or broken bones. Many travel to visit them to cure their illnesses.