Board Thread:General Discussion/@comment-26901329-20150811185125/@comment-26184894-20150811204306

I think the difference is

Spells seem to work like magical abilities. Ex- Mal casting a sleeping spell by simply waving her hand or using her staff.

Curses seem to take more than just innate magic. The caster has to gather specific things to achieve their goal. Ex-  Regina had to get hair from villains and get the heart of the thing she loved most.

They're a few exceptions but I find that it usually works that way.