Priests are the healers of Ragnarok Online. Other classes such as Monks and Crusaders also have access to the heal skill but a Priest’s skill set and buffs make them the prime healing class.
Full Support Priests are built to heal and support parties and lack the ability to kill monster on their own. There are a lot of different types of “Full Support” which range from Tanking builds to Large Party Support Builds to MVP Support Builds.
Magnus Exorcismus Priests are the Undead and Demon killers. These priests focus on the spell Magnus Exorcismus to deal damage, the down side is this spell required most of the points available and leave little room for maxing support spells. They still are capable healers but less efficient than a FS Priest.
Turn Undead Priests are often Support Priests who pick up the skill Turn Undead to support themselves solo leveling. This skill has a chance to one shot non-boss undead monsters which can lead to fast leveling at higher levels and high end gear thought TUing Anubis. TU priests still have ample room for some support skills but are forced to chose to spread themselves thin and only get partial levels in many support skills or focus on certain skills while skipping others entirely.