- See also, Magic schools
Every ability, power or spell belongs to one of seven schools, which defines what can be done to absorb, immunize against or resist it. Your resistance against five of these schools are displayed in your character information.
If you are a caster, and an opponent uses 
[Counterspell], 
[Spell Lock], 
[Kick], 
[Shield Bash], 
[Pummel], or 
[Earth Shock] on you while you're casting a spell, these and other abilities will not only interrupt that spell but also to prevent you from casting any spell in that school for some amount of time. It's good to know what schools your abilities are in so you can switch to another when this happens. It's fairly clear for Mages (the school of the spell is the same as the tree it falls under) and Warlocks (if it doesn't involve fire, it's Shadow). For other classes it's not always clear. Fortunately, the tooltip of any damage spell will specify the type of damage it does. Warriors are the only class that strictly deals only one damage type, that being Physical. Hunters and Shamans have the most varied damage types, with both being able to deal Physical, Nature and Fire damage, as well as Arcane for Hunters and Frost for Shaman.
Physical
Not presented as magic to the user, but treated as a school by the game mechanics. All classes use physical type damage, as attacking with a weapon falls into this school; the target can use Defense to avoid the attack, a shield to Block it, the Parry skill to block it with a weapon, and Armor to resist some of the damage if all else fails. Damage taken is physical damage. Most Warrior and Rogue abilities are Physical.
Some physical abilities, like 
[Demoralizing Shout] and 
[Thunder Clap], are treated like spells. These abilities have a basic resistance chance, but otherwise can't be avoided. This resistance can't be increased.
Arcane
- See also: List of arcane abilities
Arcane magic is typically utility magic that causes various effects rather than dealing direct damage that is used by Mages, night elf Priests, Druids, Hunters, and blood elves.
Most non-damaging magical Hunter abilities, including 
[Silencing Shot], 
[Hunter's Mark], 
[Misdirection], and 
[Distracting Shot] are Arcane-based. 
[Arcane Shot], however, is a main source of direct damage, as is Druids' 
[Moonfire] and 
[Starfire]. Night Elf Priests can also do Arcane damage with [Starshards]. Also, the Blood Elf racial abilities 
[Mana Tap] and 
[Arcane Torrent] are Arcane-based.
The true masters of the Arcane, however, are Mages. For Mages, the Arcane school is primarily a utility line rather than a damage line. Any spell from a Mage's Arcane ability category is of the Arcane school, including three spells that do damage ( [Arcane Missiles], 
[Arcane Explosion], 
[Arcane Blast]) and a very broad range of utility spells: Polymorphs, 
[Arcane Intellect], 
[Mana Shield], 
[Mage Armor], [Blink], Teleports, Portals, [Counterspell], and Conjured Gems, Food, and Water.
Hostile Arcane spells (including those that do no damage) can be resisted; there are items that increase one's Arcane resistance.
Fire
- See also: List of fire abilities
Spells of the Fire school typically inflict damage, which can be direct damage and/or damage over time. Mages and Warlocks use it as one of their primary source of damage, while Shamans, Hunter traps, and engineers also use the Fire school.
Many spells and environmental effects (such as lava) do Fire damage. The class most associated with the ability to control fire is the Mage. Fire is also one of the fundamental elements that Shamans can manipulate with their Totems. To a lesser extent, Warlocks can also deliver Fire damage. Warlocks specced heavily into their Destruction talent tree are sometimes called "Firelocks". Hunters' 
[Immolation Trap] and 
[Explosive Trap] also deal Fire damage; similarly, explosives and damaging trinkets created by Engineers typically do Fire damage. Mages can absorb Fire damage with 
[Fire Ward].
Like Arcane resistance, Fire resistance is a statistic that can be affected by spells and items. Paladins and Shaman can specifically increase Fire Resistance with 
[Fire Resistance Aura] and 
[Fire Resistance Totem] respectively. A Warlock's Imp can cast [Fire Shield] to increase fire resistance of friendly player characters and pets, but only the highest rank (Rank 6) will do so.
Frost
- See also: List of frost abilities
The Frost school is effective both for attack (doing damage) and defense (raising armor value). Frost spells often inflict snare or root effects, as well. Mages, Shamans, and the occasional Hunter trap uses the Frost school.
Mages also have a Frost ability category, and obviously any spell in that category is of the Frost school. Of course, Shamans' 
[Frost Shock] is of the Frost school too, as well as the Hunter's 
[Freezing Trap] and 
[Frost Trap]. Mages can absorb Frost damage with 
[Frost Ward].
[Frost Resistance] exists as a statistic as well. Paladins and Shaman can specifically increase Frost Resistance with [Frost Resistance Aura] and 
[Frost Resistance Totem], respectively.
Nature
- See also: List of nature abilities
The Nature school is usually used for nature based attacks, buffs, and curative spells. Nature damage can be direct damage (like lightning) or damage over time (like poison). This means that Rogues (poison), Shamans, Druids, and Hunters (stings) have access to Nature damage.
Acids, diseases and poisons inflict Nature damage, and damage from lightning is also considered part of the Nature school. Shamans' and Druids' healing spells are Nature spells, and other nondamaging spells such as 
[Entangling Roots] and 
[Faerie Fire] are in this school as well. Hunters' Stings and the 
[Snake Trap]'s viper poisons are Nature based, as are their 
[Mend Pet] and 
[Revive Pet] abilities. Rogues' poisons are also Nature based.
There is a Nature resistance statistic. Hunters and Shaman can specifically increase Nature Resistance with 
[Aspect of the Wild] and 
[Nature Resistance Totem], respectively.
Shadow
- See also: List of shadow abilities
The opposite of Holy magic, Shadow is probably the most versatile school. Used by Priests and Warlocks, it contains a wide variety of spells. Apart from Direct Damage and Damage over Time spells, there are various utility spells, including Drains, Crowd Control spells, and Debuffs.
Shadow damage is dealt by dark spells such as those of the Priest's Shadow Magic ability category and most of the Warlock's repertoire. The 
[Blood Pact] that a Warlock's Imp pet casts to boost the entire party's Stamina is a Shadow effect, as are the creation of a 
[Healthstone] or 
[Soulstone] and the summoning of demons. Warlocks can also absorb shadow damage with 
[Shadow Ward].
Shadow Resistance is the final resistance type. Paladins and Priests can specifically increase Shadow resistance with 
[Shadow Resistance Aura] and [Shadow Protection], respectively.
Holy
- See also: List of holy abilities
The Holy school is usually related to abilities which heal and aid living beings. There are also a few spells that do Holy damage: 
[Smite] and 
[Exorcism], for example. Contrary to popular belief, Holy Magic does not do an additional damage to the undead or demons unless it is specifically mentioned in the toolbox (such as in Exorcism).
Priests and Paladins are the primary users of the Holy school, including all their healing and resurrection abilities.
It is not possible to build resistance to Holy magic, giving Holy Priests an advantage over other casters, but all characters and mobs do have a base resistance chance. Items and Buffs which give a bonus to all resistances, such as [Mage Armor], do not include Holy resistance. Also, the damaging Holy spells used by Priests and Paladins tend to deal smaller amounts of damage in comparison to other schools, counterbalancing the lack of resistance. In PvE, Holy resistances do exist under specific circumstances (if the enemy has a higher level than you), and in that case your Holy spells may be partially resisted.
There is no Holy resistance stat, therefore buffs such as 
[Mark of the Wild] do not reduce damage from holy spells, but percentage-based damage-reduction abilities such as a warrior's 
[Defensive Stance] successfully reduce damage from Holy spells. Also, the [Holy Protection Potion], [Greater Holy Protection Potion], [Major Holy Protection Potion] items will also absorb limited amounts of Holy damage.
Additional information
A shadow Priest's 
[Shadowform] ability refers to the Priest's Holy talent tree, not the Holy school of magic. Therefore the priest can still cast holy-based spells from the Discipline talent tree while shadowformed.