Intelligence: This ability score is likened to IQ, and also defined as including memory, reasoning ability, and the capacity to learn all sorts of things not related to the written word. It's briefly noted that the number of languages a character can learn is tied to Intelligence, with a footnote that non-human characters are generally able to speak more languages than a human. (I wonder why this is. It's probably not due to them mixing more freely with other races, as generally in D&D the demi-humans keep to themselves, while humans are the ones who mix in multicultural societies. I chalk it up to their longer lifespans, and the fact that your average elf or dwarf will have lived a full human lifespan before even beginning an adventuring career. We'll see later if this pans out; do the races with longer lifespans get more languages than those with shorter?)
Intelligence remains the prime requisite of magic-users, with an XP bonus for those with a score over 16. Intelligence is also tied to the level of spell that a magic-user can learn: with a score of 9 (the minimum required to be a magic-user), the caster can only learn up to 4th-level spells. At 10 he can learn 5th level spells, at 12 he can learn 6th, and so on; only a caster with 18 Intelligence can learn 18th-level spells. (This is a change from OD&D - at least when including Supplement I: Greyhawk - where there was no minimum Intelligence requirement for magic-users, and spells of 5th level were available to every caster. Those with 11 Intelligence could cast 6th level spells, 13 could cast 7th, 15 could cast 8th, and 17 could cast 9th).
Intelligence also determines whether a caster can learn a specific spell. Each caster must go through the list of 1st-level spells, and against each one roll percentile dice to see if he is able to learn it. (The chances range from 35% with a 9 Int, and 95% at 19+). If you succeed, you have the ability to learn that spell; if not, you can never learn it. However, Intelligence also determines the minimum number of spells you can learn, and if you don't reach that total after going through the list once, you can go through again, testing spells until you hit your minimum. There's also a maximum, and once you hit that you can't go any further (unless your Intelligence is raise somehow.)
(I have to admit, I've always gotten a bit confused by the rule above. What I need to remember is this: just because I've rolled my chance to know, it doesn't mean that I know the spell already. It just means that I can know the spell at a later date, should I find it somewhere. It's not that difficult, really, but it's tripped me up in the past.)
There's a section at the end about "acquisition of heretofore unknown spells" that muddies the waters a little. I think what it's trying to say is that you can check to learn any spell you find that isn't in the Players Handbook, so long as you don't exceed your maximum number for that spell level. The wording is a little ambiguous and open to interpretation, but that's often the way with AD&D.
(Most of the above is also present in OD&D Supplement I: Greyhawk, but that book's tables provide for casters with very low Intelligence scores. The numbers aren't all that far off otherwise, except for Maximum Spells/Level; in OD&D a 17 Intelligence will allow you to know every spell of a given level, whereas AD&D requires a score of 19 to achieve this.)
Wisdom: Ah, good old nebulous wisdom. It's defined here as "enlightenment, judgement, wile, will power, and (to a certain extent) intuitiveness". You could perhaps say that it encompasses every mental ability not related to academic learning, though I'm sure I haven't thought that through well enough. It remains the prime requisite for clerics.
The table showing the various class and race restrictions based on Wisdom is an interesting one, as it says that a character with a Wisdom of 5 or lower can only be a thief. I've always found this a bit odd, especially as your mythical or archetypal thief character embodies several of the definitions given for Wisdom above. I certainly don't see why a low Wisdom precludes one from being a fighter.
In OD&D (and Holmes Basic, as far as I can tell), Wisdom had very few mechanical benefits besides granting clerics bonus XP. Now it is given an effect on saving throws against spells that attack a character's willpower (examples given include charm, fear, illusion, magic jar and telepathic attack, among other).
As with Intelligence and magic-users, wisdom can limit what spells a cleric knows. The rules are not as strict, however: it simply states that a cleric needs a wisdom of 17 to cast 6th-level spells, and 18 to cast 7th-level spells. Clerics with a high wisdom are also now able to memorise more spells than before, but those with a score below 13 have a percentage chance that, every time they cast a spell, it will fail. Nothing spectacular happens with a spell failure: the spell simply disappears without effect.