Basic Role Description
The role of a build is what you want to accomplish. To see how possible roles interact, read about fighting in groups. Here, I am going to move away from the traditional MMO roles of "Tank" and "Crowd Controller". ("Tank" has a rather specific meaning in DDO, being a defensive target for aggro). Instead, I use the terms MELEE and CASTER, and keeping the standard HEALER.
The melee's main role is to deal physical damage to the enemy. There are two main types of melees, strength based and dexterity based. Strength based melees base their build around strength, focusing on raw power. They do a lot of damage but this often comes at the expense of armor class. High armor class is possible for strength based builds but it represents a certain investment in terms of a higher dexterity, raid gear and specialized prestige classes. Dexterity based build focus on dexterity, relying on finesse and skill. A high armor class is often easily achieved but the hitting power is far less than that of strength builds.
Traditionally, the strength melee builds are based on fighter, barbarian and the paladin classes and dexterity melee builds are based on ranger, monk and rogue classes. Due to the flexibility of DDO, the link between the builds and classes is now blurred. Dexterity paladins or strength rangers are such examples. Some players have so much imagination that one can see melee wizards, bards, etc. in Stormreach.
I define caster as a role which has a reliance on spells to either control mobs or to kill them. Casters are commonly divided into nukers and crowd controllers. Nukers are casters that specialise in the use of damage spells. Nukers in general can get by with a lower score in their casting stat by carefully choosing damage spells without a save. However, most will still have a high casting stat just so they can employ some crowd control when nuking does not help. The other type of caster are the crowd controllers. Crowd controllers employ spells to restrict the fighting ability of the mobs. Some choose spells which stop mobs dead in their tracks, others turn them on their own allies. Instant kill spells are also a forte of crowd controllers. Due to the use of spells with a save, crowd controllers will often max out their primary casting stat.
Traditionally, wizards are viewed as crowd controllers and sorcerers as nukers. In recent times, due to the expanded abilities of the bard and cleric class, they too can fill the crowd controller role, with some clerics even specializing as a nuker. With the introduction of favored soul, many players have speculated how it can be played in the nuker role.
The healer's role, as its name implies, is to heal. In DDO, there are many quests where healers are not a necessity. Solid team play, careful exploration and self healing with wands and pots can allow many groups to finish quests. In raids, healers are often the backbone of the party. With raid bosses dealing large amounts of damage and fights that last longer, a healer is necessary. Healers not only replenish lost hit points, many of them are also involved in buffing and status removal to keep the performance of the party at peak. As with many other MMOs, healing in DDO can sometimes be a thankless job but many healers can walk away with the satisfaction that it is they who made it possible. Due to the nature of some quests, be prepared that the healer will sometimes need additional resources to perform its role. That said, if one is able to play it well, resource usage is negligible.
Most people would associate clerics as the standard healers in DDO. However, a well built bard is also able to fulfill the healer role. However, due to the lack of specialized spells, a bard may need more resources.