Lyrick wrote:Invisibility also lets you affect others. Charms help you, Hexes affect things.
Most spells help you in one way or another. Yes, most Charms involve "buff-like" effects, (though e.g., battlesphere is an exception), and there are buff spells that are not charms (e.g., phase shift).
Really, invisibility is a hex because charms and hexes used to all be in one huge mess (design-wise) of a spell school, "Enchantments," that was split. Haste was put in the pool of "Charms," and so Silence and Invisibility had to go the way of hexes. The others were sorted in a way that "affect others and/or more oriented toward harming others" when into hexes, while the others went into charms. This doesn't hold absolutely, however. (E.g., ring of flames is in charms, darkness is in hexes.)
I'd just define it pragmatically. "Enchantment"-like spells were too broad and thus need to be broken down. Some spells that worked well together had to be hexes to keep some backgrounds coherent, and from that a somewhat reasonable division was hashed out, but there really isn't a completely consistent, a priori principle that separates one from the other.