Hey, sup. I also enjoyed playing in the mid 80s.
AC now goes UP instead of down, and starts at 10, modified by Dex and armor.
THAC0 no longer exists, nor do the old school matrices. Instead, each class has a 'base attack bonus' that goes up as they level. Fighter types increase by one every level, clerics/rogues every three out of four, and mages every other level.
You add this number, your STR bonus (or dex if ranged), and any other modifiers like weapon bonuses to a d20 roll. If you roll equal or higher than the AC you hit. So much better than the old way.
For every +5 you have in BAB, you get an extra attack, so at 20th level fighter types get 4 attacks per round, clerics 3, mages 2. This is assuming they do nothing else.
There are now things called 'feats' which are selected every third level (fighters get bonuses from a selected list as well). These allow you to do things like increase you movement rate, make an extra attack against an adjacent foe whenever you kill something (like a follow-through attack), or modify the parameters of a spell you're casting.
Skills are completely different that the old non-weapon proficiencies, which I loathed. Each class gets a certain number of points each level that they can spend on 'ranks' in skills. These skills are checked by a d20 roll + number of ranks + bonus from relevant stat (the skill 'Spot' has Wisdom as it's relevant stat) vs. a target number for whatever is being done.
Yes, you can still get modules, though you'd have to do a good bit of work to convert them backward. WotC (the Magic people) bought TSR when they were about to go belly up, and basically made it possible for any company that wished to publish 3rd edition D&D material, which is a fairly large about face from the old TSR policies. There is a shitton of material out and on shelves (some great, some good, some bad, some VERY BAD) including sourcebooks and adventure modules.
In fact, you can actually get the rules for
FREE RIGHT HERE if you'd like to give them a read. Stuff like deity names and character names (like in spells - Melf, Mordenkainen, Tenser, etc.) aren't in there since they're reserved for WotC's use, but the spells are there without the names (Acid Arrow and the like)
I heartily recommend stuff from Necromancer Games, as those guys really make stuff that feels like 1st edition AD&D using the 3rd edition ruleset. If you remember the old Judge's Guild stuff, they're re-releasing the Wilderlands and City State stuff right now.
If you ever played in Forgotten Realms, it's now a huge clusterfuck and seemingly every bartender is a level 14 character. Power Creep really has set in over the years, but the quality of the sourcebooks that come out for the setting is still top notch.
Greyhawk is still around, but it's pretty much abandonded by WotC. They however, handed the world over to a player-run worldwide Living campaign,
edit: oh yeah, you don't have to color the numbers on your dice with a crayon anymore