Arcane spells are damaging spells that deal damage to multiple enemies without any additional effects.
Arcane Ray is a beam-type spell that deals damage over time to all enemies standing inside the beam. Its damage ticks are low and it prevents you from casting during the time the spell is active.
Like all ray-type spells, it (currently) ignores wand adjustment.
Arcane Explosion damages multiple enemies in a large area, and is the only AoE runemage spell in the game. It is sometimes mistakenly called Arcane Blast or Arcane Missile.
With the True Affliction talent, the explosion spreads fire, frost, and affliction DoTs on enemies to the rest of the targets. Only one debuff of each type can be spread, and debuffs spread by Arcane Explosion don't stack.