In the 2nd Edition of the AD&D rules, the concepts of wizard specialization and religious spheres of influence were introduced. Without going into excruciating detail (and probably end up copying a good section of the Players' Handbook), I'll give a precis to cover the basic concepts.
Wizard's spells are broken up into different categories according to differing criteria. Spells fitting similar criteria are considered to be of the same school. Wizards are now allowed to specialize in specific schools, much the same way doctors may specialize in specific areas of medicine. Like doctors, specialization brings a high degree of competence in the chosen field, at the cost of neglecting others. As such, the Illusionist class no longer exists; it has been absorbed into the Wizard class as a specialist in Illusions.
Mages may specialize in one of three ways: in school magic, in elemental magic, or in wild magic.
All school magic specialists can learn Lesser Divination spells. The following diagram shows the relation of the other schools to each other:
| Alteration | Illusion | Enchantment / Charm |
|---|---|---|
| Greater Divination |
| Conjuration / Summoning |
| Evocation / Invocation | Necromancy | Abjuration |
The school directly opposite a given school is considered its opposition school.
Similar to school magic, elemental schools have opposition schools:
| Smoke | Fire | Magma |
|---|---|---|
| Air |
| Earth |
| Ice | Water | Ooze |
A wizard who does not chose to specialize may pick his spells from the entire list, irregardless of the school it belongs to. They obey the rules normally. Specialist wizards, on the other hand, have several benefits and penalties:
In the 2nd Edition, priests are longer general-purpose characters, and their gods are much more narrowly defined. Each god has a proper place in the parthenon it belongs to, with duties, responibilities, etc. Consequently, there are certain areas that the priest's deity is critically concerned with, and others that are of little concern or even anathema to the deity.
Priest spells are now broken into spheres of influence; similar to school magic, a sphere is a grouping of spells that have a common purpose or concern. The following spheres of influence are defined in the Player's Handbook and Tome of Magic:
Spheres may concern a variety of things, and a deity usually has several favorite spheres. The deity may freely grant the priest spells of any level from the spheres directly under its control. A priest may request spells from spheres that the god may be peripherally concerned with, but there may be some level limit or total number limit of spells bestowed. Certain spheres are alien or opposed to the god, and requesting spells from them probably will result in considerable conflict with one's deity.
In addition to spheres, the Tome of Magic introduces cooperative magic, faith magic, and quest spells. See the aforementioned work for a complete discussion on these topics.
For complete information of magic specialization and metamagic spheres of influence, see TSR's The Player's Handbook, 2nd Edition and The Tome of Magic.