Wis casters, Clerics, Druids and Shamans, use wisdom as their spell casting statistic. The base wisdom, as modified, determines the mana pool for their spell casting ability. They can wear different types of armor and have different abilities, but all share the singular abilities to heal and cure. They also have differnt types of buffs. Their magic is basically defensive in nature, though they do have a range of offensive spells.
Clerics are probably the single most powerful healers in EQ. They also have a narrow range of buffs, though these are quite impressive. Because of the agro they can pull, and the fact that they are often associated with warriors, they are allowed to wear plate armor.
Druids are probably the weakest pure healers in EQ, though if the class is known they can be quite impressive healers, sometimes doing better than clerics (though this is rare). They also have a line of spells that can stop an enemy mob in its tracks and "root rot" it to death. Druids make great soloers but are limited to leather armor at best.
Shaman are inbetween Clerics and Druids in healing ability. They also have the most impressive list of buffs available as well as a limited form of crowd control. Their hastes and slows, however, is what sets them apart, they can haste the attack speed of fellow party members and slow the attack speed of enemy mobs. As such, they are probably the most effective healers. They can wear up to chain armor.
There are three wis caster hybrids. Originally there were two, the Paladin (a hybrid of the warrior and the cleric) and the Ranger (a hybrid of the warrior and the druid limited to chain armor and capable of mastery of archery). These utilize wisdom as the basis of their spells just like the regular wis casters do. Laser on, the beastlord came into being, it is a hybrid of warrior and shaman, limited to leather armor and uses a warder, an intelligent pet, that assists the beastlord.