A spell or enchantment is a specialized technique cast upon an object, place, or living being with the purpose of either activation, empowerment, or enhancement. For instance, a spell might be used to keep a waterwheel turning eternally, irrigating a field for a thousand consecutive years without pause. Another might be used to amplify the defensive capabilities of a shield, making it resistant to fire for an entire century. These are but simple examples...
On the other hand, a curse is something entirely different — it is a malevolent force, hurled with the intent to corrupt, to harm, to doom. A Curse of Eternal Stillness that freezes the soul in place forever. A Curse of Pain and Decay, like the one that struck Roben from within the Soul Tomes. Or perhaps a city-wide curse — one that repels all good karma, attracts relentless misfortune, and dooms the city to a slow, inevitable ruin until it crumbles into dust.