The simplest of magic in the world, arcane magic is a blunt force instrument for forcing ones will onto the world. Arcane magic does not come freely, and the energy of such magic must come from somewhere, with many practitioners of arcane magic being forced to find an energy source such as an arcane battery or blood magic. With the constant need for power to fuel the magic, many of the more powerful  practitioners of arcane magic fall to darkness. 

Divine magic is the magic of the gods, both Valar and Maia, and those who utilize divine magic are simply allowing the gods' will and magic to flow through them, acting merely as a conduit. A common source of magic for clerics and druids. 

Runic magic is the original magic of the world, with the various runes being the lyrics by which the world was sung into existence. Using runic magic does not just alter what something is in the moment, but what it always was and always meant to be, harking back to the original chords. The most unknown of the magic sources, really only used by the Iron Lords in the realms of men

Eldritch magic is the only magic source that does not draw in some way from creation, but rather from the endless void, a force of corruption and nullification. Only the most insane, or inquisitive and willing to risk their very beings will pursue eldritch magic.