A "class" in Terraria is a specific build that utilizes armours and their set bonuses, and their specific weapons with the damage type to augment a playstyle.
Like how Necro Armor is for Rangers who do Ranged Damage, and Bows do Ranged Damage.
And Fossil Armour or Ninja Armour is for Throwers who do Throwing Damage, and various consumables like the Shuriken deals Throwing Damage.
And the Spooky Armour is for Summoners who uses Summon weapons that deal Summon damage.
In Vanilla the classes are: Melee, Ranged, Magic, Throwing, and Summon. They all play differently.
So now in ThoriumMod, in addition to the Vanilla classes, you have the Bard class who deals Symphonic Damage, and the Healer class who deals Radiant Damage.
EDIT: The Healer, while still able to deal damage, is severely gimped in terms of damage but makes up for it with powerful healing abilities for other players. They directly restore the health of other players, mostly.
The Bard, new addition, buffs themselves and other players whenever their weapons hit enemies. The buffs are short-lived but they're able to sustain it as long as they attack. They also have weird "Huge Instruments" that they can deploy on the field, like the Summoners' turrets--except it only works when the bard is near it (playing it) and that they do a variety of effects like buff Movement Speed of the Bard and and his allies.
