Magic (PHB)

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Some of the most powerful weapons player characters have at their disposal in the AD&D game are magical spells. Through spells a player character can control earthquakes, call lightning out of the sky, heal grievous injuries, hurl explosive balls of fire, create barriers of stone, fire, and ice, and learn secrets long forgotten. These are only a few of the things player characters can do once they master the strange lore of spells.
Not every character is capable of casting spells, however. This ability requires a certain amount of aptitude, depending on the type of spells cast. Wizard spells are best mastered by those with keen intelligence and patience for the long years of study that are required. Priest spells call for inner peace and faith and an intense devotion to one's calling.
The vast majority of people in a fantasy campaign lack these traits or have never had the opportunity to develop them. The baker may be a bright and clever fellow, but, following in his father's footsteps, he has spent his life learning the arts of bread making. There has simply been no time in his life for the study of old books and crumbling scrolls. The hard-working peasant may be pious and upright in his faith, but he lacks the time for the contemplative and scholarly training required of a priest. So it is only a fortunate few who have the ability and opportunity to learn the arcane lore of spellcasting.
A few character classes have a limited ability to cast spells. The ranger, through his close association with nature, is able to cast a few spells, though his choices are limited to his natural inclinations. The paladin, through his devotion and humility, can use some of the spells of the priest. The bard, through luck, happenstance, curiosity, and perseverance, can manage a few wizard spells, perhaps by persuading a lonely wizard to reveal his secrets.
Regardless of their source, all spells fall into the general categories of wizard or priest. Although some spells appear in both categories, in general the categories differ in how spells are acquired, stored, and cast.