On the Categorization of Magic:

Almost all magic can be classified into one of two groups.  The names given
to these groups are varied.  Some have called them the Chaotic and Ordered
groups; while this is technically accurate, it is confusing, as most fire
magic falls into the group described as "Ordered", while many protective
spells are considered chaotic.  Frequently they are referred to as Old
Magic and New Magic, reflecting the fact that Old Magics are considerably
harder to create and to learn, and thus must be handed down by generations
of wizards.  The terms we shall use are External and Internal magics, to
describe the sources of power used by practitioners of these magics.

The distinction between the two types of magics is best shown by example:
  Example of External Magic:
    "If I chant the four mystic runes while throwing dried oak leaves into
      the air in front of me and gesturing with my enchanted athame, then
      that door will open."
  Example of Internal magic:
    "If I use my telekinetic powers to open that lock, then push on the
      doorknob, then that door will open."

"Internal" magics draw power from the user's magic or energy reserves.  They
are controlled directly by the user.  They need not be memorized, and no
special ingredients are required; you just throw the right kind of energy
at something, and it works.
"External" magics, on the other hand, tap into the fundamental chaos that
underlies reality.  Special ingredients, ritual gestures, unpronounceable
magic words, and the like are usually needed to trigger the effects.
These spells must usually be memorized from books, cast, and re-memorized.
They are much less adaptable than Internal magics; any change in the casting
of an External spell results in wildly unpredictable, and often quite
destructive, results.  On the other hand, External spells are far more
versatile, as there are a lot of things that Internal magic just can't do.

For example, it's very difficult to affect something you can't see via
Internal magic, not because the forces won't coalesce or anything silly
like that, but because the caster will probably miss.  Divination spells
are completely impossible with internal magic.  Flight is not impossible -
that's a fundamental restriction of wizardry, not of internal magic in
general - but it's difficult, especially compared to Triolus's Ethereal
Levitation, a third-level Old Magic spell that accomplishes the same
objective at a cost of only two hen's teeth and a leaf of nightshade.
Furthermore, there are intrinsic restrictions on the power of any Internal
magic user; only so much energy can be stored in any one body, and it can
only be regenerated so fast.  Old Mages are restricted only by the
availability of magical spells and components, and by the rate at which
they can memorize new spells.

What is the practical application of all of this?  Wizardry, the
predominant magical discipline on the world of Dirt, is a strongly Internal
discipline.  Most of its spells rely on throwing power around, and, as
such, it is extremely popular.  However, there are times when one really
needs a divination, a long-distance assassination, or a polymorph spell.
Providing these resources are the Clerics, the followers of the
planeswalkers, the source of most of the Old Magic on Dirt.

  A partial ordering of the magical disciplines in Dirt:

     Warlockry                    INTERNAL
     Elementalism
     Witchcraft
     Wizardry

     Artificing
     Sorcery
     Illusion
     Psionics

     Herbalism
     Che'tai training
     Jordanism
     Necromancy
     Thaumaturgy
     Clericry                     EXTERNAL