The five spell school skills of abjuration, alteration, conjuration, divination, and evocation form the backbone of spellcasting in EverQuest.  All casters, priests, and hybrids (except bards) possess all five of these skills.

Mechanics

Casting a spell from a given spell school has a chance to raise the caster's skill in that school, up to a cap based on the character's class and current level.

Improving one's skill in a spell school decreases the likelihood that spells from that school will fizzle.

Strategic Impact

Trying to cast higher-level spells with too little skill in a school can result in repeated fizzling, wasting both time and mana.  As such, building up one's proficiency in less-frequently-used spell schools (by deliberately taking time to cast such spells every now and then while one is leveling up a character) can be wise.

Some classes almost never use certain spell schools beyond their lowest-level effects, and can therefore safely pass up raising those schools much.  A few of these cases are listed below:

• Classes other than enchanters use Divination only infrequently, or only for very low-level spells, and generally only "need" enough skill to keep their current invisibility spell from fizzling.

• Paladins never use Conjuration for anything other a few low-level item conjuring spells and their level 55 and 70 aura spells.  Getting the school's skill high enough to cast the aura without excessive fizzling is therefore sufficient.
• Shadow Knights never use Evocation for anything other than some of their undead bane spells, which stop being upgraded after the level 66 to 70 range.  Since the class can deal similar damage to any type of creature using both Alteration and Conjuration spells, ignoring Evocation is entirely feasible.
• Rangers Use Evocation for Direct Damage, Conjuration for Dot, Abjuration for buffs. Generally speaking