The AD&D core rulebooks: Monster Manual, Players Handbook, and Dungeon Masters Guide. DMG seen HERE
The AD&D2E core rulebooks: Monstrous Manual, Player's Handbook, Dungeon Master's Guide. PHB seen HERE Just as with AD&D, the core rules were also released with alternate covers late in the run, in this case, black bordered covers. AD&D2E had the distinction of using a binder format for the original MM, which was abandoned in favor of the hardback book.
There is a lot of confusion on nomenclature for the original and non-AD&D lines. My personal preference is OD&D (for Original D&D - the box sets starting 1974 with the 3 'little brown books', seen HERE); and Classic D&D (seen HERE, begins with the Holmes-edited Basic box set of '77; the revised Basic and 'new' Expert set edited by Moldvay/Cook, oft called B/X; and the revised set edited by Frank Mentzer, or BECMI for Basic-Expert-Companion-Master-Immortal; then we have the Rules Cyclopedia, which was a revision of the BECMI set put in a single hardback volume).2. Are OD&D and 1st the same thing? What are the core book(s) to play called?
OD&D is a different beast on some levels. It first assumed the reader had a background in tabletop wargaming, which many have difficulty grasping. Things like no variable weapon damage, all hit points being based on a d6, and classes limited to cleric, MU, fighting man (and elf, dwarf, and hobbit/halfling), will likely be quite strange to someone coming from a later rule set. There is overall much less reliance on rules, and more emphasis on rulings. Later supplements, notably Greyhawk, added more classes, and gave variable hit points for classes and weapons, bringing the game closer to what we recognise as AD&D.
The Classic line firm. OD&D was a bit iffy, from the standpoint that all dwarves were 'fighting-men', while all elves were both 'fighting-men' and 'magic users' freely able to change from one class to the other on any given adventure. The supplements began to solidify things more towards AD&D separations.3. which one of these two had non-human races as classes? both?
Generally speaking, most things from AD&D and AD&D2E are interchangable, as are things from the Classic line. Some things will need a little modification.4. Are 1st and second edition very dramatically different? like 3rd and 4th were?
I personally find there are two things which stand out the most: Firstly, there is more reliance on rulings over rules in earlier incarnations. Secondly, and the big reason I have gone back to prior games as my preferred choice, is that the later one goes, the more I've noticed the game becoming one where the adventure is an outlet to explore the character, whereas earlier, the character is an outlet to have an adventure.5. Can anyone give me a general feel or idea about these systems to help me decide if I'd like them?