That's a really really long explanation that will be elaborated in the OOC, but I'll give a simple answer right now:

Magic works in the world like it does in typical RPG games; it mostly centers on elements, like summoning a fire ball or creating an ice storm. In order to use magic, it's channeled through tomes. A mage can't use magic without specific tomes (like Harry Potter can't use magic without his wand). If a mage wants to use Fire magic, they would need a tome on fire magic in order to read spells and use it to channel energy. 

Magic is considered bad mostly because of its origins, which are taboo and shrouded in darkness (will be explained in OOC). People thus think of mages as people who have connections to demons and other evil entities.