Magic is fairly simple business, theoretically. Conceptually, magic is conversion of mana to energy. Mana is the "energy" that binds your soul to your body. Your soul, in turn, is basically your mind. So to cast magic, you must "think" in a way that moves mana from the spiritual plane to the physical. The energy will be manipulated in the manner you will.

There are a few difficulties with this. First off, you must dictate the spell precisely, like a programmer, because raw energy has no interpretive power. Secondly, our thoughts are all over the place, and attempting to cast a spell without focus yields unpredictable results.

There are a few ways to handle these issues. Speaking forces you to focus, so most mages chant their spells until they get good at focusing. The precision problem is rectified by soliciting the aid of an intermediary interpreter, usually a spirit of some sort. Soliciting spiritual help carries its own risks, though, so modern mages have devised a better solution - tomes. With a tome, you can simply look up the spell you want and read it. While the tomes themselves have no power of their own, the knowledge they carry is painstakingly researched and often proprietary, so they are closely guarded.

But there is another level of magecraft that deals only in mana and never touches energy conversion. It is purely spiritual, and deals in things like curses. Some go even further and cast their spirit around. Any non-converted mana manipulation is labelled "dark magic," mainly because it is generally unresearched and rarely has any positive/beneficial uses.

The spell that the enemy cast on the party is primarily dark magic, since it only locks down mana production and verbalized thought within a specified area.