Here are my thoughts on what guardian magic really is. I wrote it a while back on the german rp forum, heavily edited and translated.
It's hard to define what magic really is. Therefore I'll just start by describing the guardian's school of magic by highlighting differences to other disciplines.
It is easiest to see when compairing guardians to elementalists. Elementalists look outward and source their power from their surroundings, the destructive or restorative powers of the elements. The guardian's source of power however is found within the practitioners themselves, within their souls, within their faith. Guardian spellcraft is a spiritual but not a necessarily intellectual discipline. Albeit traditionally religious (monk, paragon, ritualist) the derivated new form doesn't need to be that anymore, as one can see with 'atheist' asuran and charr guardians. When I speak of spirituality I primarily mean wisdom and awareness of and mastery over one's mind and beliefs, at the very least it's utilization.
This power prioritizes willpower, dicipline and self-control over intellect, if latter contributes to the equation at all. Neither Logan Thackeray, nor Braham, nor the charr companion Dinky can be called intellectual powerhouses after all.
'External' and 'internal' is of no concern to the necromancer. He uses life force wherever he finds it. He collects the life force of surrounding beings - preferably his enemies' - or he taps into his own reservoir. To spite those who wronged him (spite); to spread pestilence, satisfying the hunger of lesser lifeforms; to negotiate, even rule over life, death and entities of the afterworld as a dark pact maker (death magic); to cling to immortality (blood) or to accept death and be the tool and envoy of the reaper.
Thematically I fancy drawing up the following opposite archetypes: The necromancer is a materialistic, nihilist egomaniac who calls upon darkness, coldness and concentrates on the body's life force - standing in stark contrast to the spiritual, moralist servant of the common good, who wields divine fire and radiant light constructs, only ever interested in honing the virtues of his spirit in order to fulfill his responsibilities.
While that may work, there are many more archetypes hidden in the profession of the necromancer, one of which (the grenth priest who speaks to dead souls for example) has mastery over spectral skills. There is a partial overlap, it's true nature still eludes me.
Both - guardians and mesmers - use their spirit as a tool. However, whereas the guardian strives for sheer, unshakable willpower, forging and tempering his soul into a mighty hammer with which he smites people, the mesmer uses trickery, sleight of hand and arcane knowledge, honing his spirit into a sharp razor with which he sticks and prods cleverly into whatever opportunity, be it the mind of the opponent (domination) or vulnerabilities in his defenses (dueling), weakpoints in the fabric of reality (chaos), take inspiration and adapt phantasms or confound onlookers with mesmerizing illusions.
Guardian and mesmer overlap thematically with their light and mental aspects. However the guardian looks inward, achieving mastery over his soul via dicipline and faith, enforcing order, stability and structure in the world and upholding truth with his voice, whereas the mesmer has his attention outward, achieving control over the situation with intellect and instinct, playfully deploying controlled chaos and unraveling entropy, finding, interpreting and conveying unorthodox truth by way of deceit, fiction and imagination.
I summarize: The archetypal guardian is a disciplined, introspective, moralist soldier wih average intellect who learned magic sourced from his soul, his faith, using sheer willpower, casting spectral light and fire spells, guided by higher values called virtues.
The guardian finds himself in a different situation than most casters. While other disciplines have the option to opt for melee, the guardian in its core is pre-defined as a frontline combatant. He has to physically and mentally react to opponents offending him with strikes and spells which occupies concentration, making casting complex spells rather difficult. Casting spells was once likened to playing a music instrument. Imagine playing a piano with your left hand, trying not to mess up beat and sequence while you had to keep your eyes on the enemy, maintain enough situational awareness so you won't get flanked and fence with your right sword hand. Casting spells therefore had to be intuitive. Maybe another reason why the spells have names that hint at (religious) rituals and concepts, why belief is so central. Ritualized practices, early morning recitations, passages learned by heart, associative battlecries, (religious) symbolism, the footing in one's convictions, all of a sudden those things have a more practical, rooted meaning. Necessity.
Speaking of necessity. The guardian has two types of weapons. One of steel and one of spirit. The difference between them being, that steel is reliable whereas an untempered spirit is not. Pain, fear, indecision, distraction, complacency. The spirit is fleeting and bends. A guardian needs to be trained accordingly, putting focus on the tempering of the soul. Make it as reliable as steel.
Lastly a few thoughts on how exactly guardian spellcasting works. Everyone needs to orient themselves in the world, valuing some things over others while on the other hand ignoring matters, that are deemed irrelevant. Everyone has to make hard decisions based on that, everyone has a world view. However the guardian is the sole discipline that furthermore also gains strength when his convictions align with his decisions, when he believes he has made the right judgement call and is resolved to carry it out to the end, which places exceptional meaning on the guardian's decision-making and his perceptions. While this generally orients him to truth, it also leaves some leeway for (willfully) blind guardians with rigid thinking, who want to remain 'pure', secluded from corrupting thought and effects. I think its at this point virtues carry significance. We see exactly those virtues we see ingame because they carry him through this decision-making process.