Acolyte is a base class focused on healing, buffing, supporting oneself and others and fighting demon and undead monsters. There are essentially two paths when playing as acolyte: going maximum support and undead-killing with high intelligence and vitality into a Priest or building as a self-buffing battle class that will eventually change into a Monk. Regardless, with so many useful acolyte skills changing class at Job Level 50 is practically mandatory in all cases.
Important skills: Heal, Agility Up, Blessing, Pneuma (all builds), Angelus (Support), Divine Protection lvl 10 and Demon Bane lvl 10 (Monk)
Priest
The ultimate party support and healer. With high Int, Vit and some Dex you have great healing, solid survivability and good casting time to keep your teammates buffed and healed with minimal SP downtime. Even if by some miracle you don't have people wanting to party with you you can still solo level by healbombing undead monsters and exorcizing powerful undead like Anubis(when he's added). High vit is mandatory for PVP and War of Emperium as you will be targeted by virtually everyone of the enemy team.
''Important skills": Impositio Manus, Ressurrection, Magnificat, (all builds), Kyrie Eleison (Priest) OR Assumptio (High Priest), Turn Undead (Exorcist).
Monk
Unlike the support-oriented Priest, the Monk is a more rounded and self-sufficient fighter with some supernatural abilities of his own. Monks have a wide variety of skills available - from high damage combo attacks to the close-ranged Investigate which deals higher damage the higher the defense of the foe to freezing attacking foes with Blade Stop and their most famous skill - the incredibly powerful Asura Strike. The downsides to being a monk are the very limited SP, limited support abilities and the fact that many of your skills are forced neutral damage, which makes it difficult to deal with certain foes and builds.
There are two paths one can go when playing as Monk/Champion: agility-focused combo builds that deal good sustained damage and can easily dodge enemy attacks (and are fun to boot) to dexterity-focused builds that rely on their spirit skills such as Investigate and Finger Offensive as well as Asura Strike. Regardless, you will want very high strength on both counts for maximum damage.
Important Skills: Summon Spirit Sphere, Investigate (All Builds), Triple Attack, Chain Combo and Combo Finish (Combo), Finger Offensive, Asura Strike (Spirit), Dangerous Soul Collect and Body Relocation (Champion).