.@yuvalaziza Both priests and paladins can wield the Holy Light. However, not all wield it through the same means (e.g., Elune, An'she). :) - Sean Copeland (@Loreology) July 22, 2013
These types of questions get asked a lot, however. And while not everything regarding magic in Warcraft has been fully defined, we can definitely take a look at these different schools of magic and how they relate to one another on Azeroth.
Mages and the Well of Eternity
When the Kaldorei began to experiment with the Well of Eternity, they were delving into the arcane arts -- and the Well was powerful enough that manipulating its energies caught the attention of the Burning Legion. Of course, the War of the Ancients saw the Well meet its inevitable end during the Sundering, imploding and merging with the waters of Azeroth's oceans to create the Maelstrom. But that wasn't the end of the Well. Illidan Stormrage saved several vials of water from the original Well, and it's been used to re-create this font twice over now -- once on Hyjal, and again for the Sunwell.
When the high elves encountered humankind during the Troll Wars, they offered to teach a section of the humans how to practice magic in exchange for the human nation's help fighting off the trolls. That's how humanity learned how to use the arcane -- they were taught by elves. Years later, these humans and high elves, along with some inquisitive gnomes that displayed a pretty good affinity for the arcane, formed the city of Dalaran as a haven to continue their studies and research.
While that's all well and good, mages are most definitely not the only wielders of magic on Azeroth -- magic users are pretty common. It's the schools of magic they practice that differentiate them.
The arcane is typically used to refer to the schools of magic practiced by mages. It's just raw magic power, scattered over Azeroth in the form of ley lines that mages can use to draw power from. While it's never really been sketched out in detail, I like to think of the arcane as magic in its purest form -- it's not influenced by nature or dark forces, it's simply there to use and wield, whether that be for bad or good. Other schools of magic take their influences from other forces, while arcane magic is simply ... magic, with no other real definition.
Some may point out that mages can use elementals and elemental power in the form of fire and frost spells -- but I don't think this is really elemental magic. The difference between the elemental magic used by mages and the elemental magic used by shaman lies in how it's obtained -- while shaman revere the elements and work with them, mages simply harness their power and use it. In Tides of War, Jaina decides in her grief to summon thousands of water elementals to drown Orgrimmar. In a panic, the elementals call out to Thrall, concerned and frightened with the power being demonstrated.
To me, this suggests that the frost and fire powers that mages wield aren't a result of a direct connection with the elements -- rather, it's the use of arcane magics to harness elemental power. Mages don't worship the elements, they simply use their arcane prowess to control and wield elemental magic according to their whim -- not the whim of the elements. With mages, all of their power seems to trace back to that original affinity for the arcane.
But then, you ask, what about warlocks? They're also using the powers of fire, aren't they? Yes and no. Fel magic is sort of a form of arcane magic, but it's arcane magic that's been corrupted -- typically the kind of magic you see used by demons of the Burning Legion. The roots of warlocks lie with the Burning Legion itself, beginning with the orc warlocks of the Shadow Council, who were taught their dark powers by agents of the Burning Legion. The practice later spread to other races on Azeroth -- whether due to wanting to fight fire with fire, or simply individuals who were really impressed with the magic that warlocks wielded.
The biggest difference between arcane and fel is that inherent corruption that lies in fel magic, and that corruption often manifests itself over time. The green skin of the orcish race is directly tied to their corruption by the Burning Legion and the influence of fel magic, the green eyes of the blood elves are also similarly tied. The Broken, a race of devolved draenei, were twisted by the use of a "fine, red mist" the orcs unleashed on Shattrath City -- likely another form of fel weaponry that twisted the race into their current devolved state.
The other big difference between warlocks and mages is the use of demons. Because warlocks have a tie with fel magic, they are cut off from the elements -- many of the original orc warlocks were shaman that had been shunned by the elements. Instead, warlocks harness demonic entities from the Twisting Nether and force them into servitude. So what we're looking at here are two forms of arcane power -- mages use the pure form, warlocks use the corrupted form.
Shadow magic and necromancy
Are the shadowy arts practiced by warlocks and the shadow spells cast by priests the same thing? Not at all. To understand the arts of shadow priests, we have to first look at necromancy -- the study and use of magic to raise, manipulate and control the dead. Some of the orcs of the First War dabbled in necromancy, encouraged by Gul'dan and Kil'jaeden. These necrolytes were observed on Azeroth by a bishop in the Church of Holy Light, Natalie Seline. While she originally sought to unravel the works of necromancers in order to better understand and fight them, she soon grew fascinated with why these energies existed at all -- why she saw these energies ebb and flow in the world around her.
In short, it was all about balance in the world to Natalie -- the balance between Dark and Light. Eventually, Natalie died, her works and research locked away. In later years, her works were re-discovered by Forsaken priests, and the Cult of Forgotten Shadow was formed. To Natalie, and to the practitioners of Shadow magic, the dark energy of the shadowy arts was just as vital as the practice of the Light -- it was all part and parcel of the balance of reality itself. Other races use Shadow magic as well, though whether it's from tales of the Cult of Forgotten Shadow, or a simple indulgence in the balance of Light and Dark has never really been defined. Once thing's for certain, though -- it has nothing to do with warlock magic.
Nature magic is a different kind of thing altogether. Shaman, as mentioned above, possess a strong connection to the elements of the world. The elements are simply a natural force that make up a world -- they existed once the world came into existence. They're part of the natural cycle of life -- that's why elementals exist on other worlds. Outland, although shattered, was once Draenor -- and the elements in Nagrand exist just as the ones on Azeroth -- a part of the weird natural forces that make a planet, a planet.
Shaman have that unique connection to the elements of the natural world, but they also possess a connection to the spirit world as well. The powers that a shaman wields aren't a result of control, they're a result of asking the elements for aid -- and the elements deigning to answer. Elemental unrest affects shaman almost immediately, because an element in a state of unrest or distress is unlikely to answer that call for aid or assistance. Hand in hand with the tie to the elements is a tie to the spirit world and a deep connection to the ancestors that have already passed on.
Druids, on the other hand, harness natural magic in an entirely different fashion. History states that Malfurion Stormrage was the first druid on Azeroth, taught by the demigod Cenarius, son of Elune. Druids draw their power from nature itself -- not the elements, which comprise part of that natural balance, but the actual raw energies of nature. Because of this, druids are able to shapeshift into several different animal forms. Shaman can shift into the form of ghost wolves -- but the fact that they are "ghost" wolves suggests this might be a gift from the spirits, rather than a result of harnessing raw nature power.
Holy magic and the Light
The Light -- sometimes referred to as the Holy Light by certain races -- is a benevolent school of magic typically used to heal or cleanse. It can also be used offensively, but the mastery of the Light is one of those weird things that takes several different forms depending on what race you happen to be looking at at the time. For the human race, as well as the dwarves, the Holy Light is tied up in religious practices with the Church of Holy Light. For the draenei, they were taught the use of the Light by the naaru, strange beings seemingly composed of the Light themselves.
And then we have the peculiar nature of the Sunwalkers, and the priestesses and priests of Elune. The tauren tell stories about the Earthmother, and the eyes of the Earthmother -- An'she and Mu'sha. Over the years, the tauren stopped speaking of An'she, and instead referred to Mu'sha almost exclusively -- likely because of the kaldorei, who called Mu'sha something very different -- Elune. In tauren culture, An'she represented the sun, while Mu'sha represented the moon. During the war in Northrend, two tauren, Aponi Brightmane and Tahu Sagewind, discussed the peculiar absence of An'she in tauren culture.
That brought about the formation of the Sunwalkers, who follow the Light of An'she -- very similar to the Holy Light practiced by the Church of Holy Light. Similarly, the powers of Elune harnessed by the night elves are often referred to as "holy" powers, suggesting they are Light-based as well. In fact, the powers wielded by priests and paladins on Azeroth and beyond all look remarkably similar -- they all resemble different forms of the Light.
Differences in the Light
Maybe all of these assorted races are drawing their Light-given powers from the same source, maybe not -- that's never really been defined in lore. What we do know is that basic output, that school of magic, the actual result of casting a priest or paladin spell -- it's all Light-based in appearance. According to Sean Copeland on Twitter, priest and paladins all wield it -- but the difference lies not in the magic itself, but how they access and wield that power. Sean later pointed out that for game-related reasons, the appearance was the same, but in lore, it might look a little different.
So here's the deal -- we don't know where the Light comes from. It might all come from a single entity, it might be taught by several different entities who have learned how to master it over time. What the Light is, what it means and how to master it differs between races. For humankind, mastery of the Light resulted in the development of a Church that practices three virtues -- respect, tenacity, and compassion -- in order to harness Light-given powers. If those three tenets are not upheld, the Light simply refuses to work for them, resulting in what many refer to as the Light "turning away."
For the Sunwalkers, they have their own distinct set of rituals revolving around the light of An'she, who they view as the sun. An'she may or may not actually be the physical sun that rises and sets in the Azeroth sky, some sort of benevolent god of Light, we honestly have no real idea -- but the sun is what the Sunwalkers have chosen to identify with as a god of sorts. We don't really know the rituals of the Sunwalkers, that lore hasn't been defined yet, but there is a definite tie with the worship of An'she.
For the night elves, the Light has taken a very different face -- that of Elune. Elune represents the moon, has been referred to as a moon goddess, depicted as a night elf woman, but never been physically seen. Are Elune and An'she the same thing? No. Are they naaru? No -- I may have theorized on this once upon a time, the CDev department may have cheekily inferred they look similar according to Velen, but they have never been defined as such in lore. It's not canon at all.
What all these different deities and practices, be it the Church of Holy Light, the worship of An'she, or the followers of Elune have in common are simply what they offer to the mortal races of the world -- the use of a benevolent power. While we don't have a distinct definition for what the Light is, or its actual origins, we can infer from lore over time that whatever that power is, many, many races across the universe have come to respect and revere it -- and share it with others as well.
The casters and spellcrafters of Warcraft are as varied and diverse as the worlds on which they inhabit -- but the base powers of each type of caster suggests that magic is a common thread that holds the universe together. Whether used for good or ill, corrupt or benevolent, it's clear that magic is just as important to the mortals of the scattered worlds as any other commonly found resource. It can be a weapon, a tool, or an instrument of mercy -- the ultimate choice lies in the hands of the caster.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.