No. OD&D is a somewhat inexact term, but there were versions of the game called just "Dungeons & Dragons" that preceded AD&D. Early in the life of this D&D (what is now called OD&D), a separate version, Advanced Dungeons and Dragons was launched. The two lines D&D and AD&D co-existed for a long time. AD&D and AD&D 2e each had a Players Handbook, Dungeon Masters Guide, and Monster Manual (2e called it the Monstrous Compendium and Monstrous Manual). Most versions of what you're calling OD&D had, at a minimum a PHB and DMG.2. Are OD&D and 1st the same thing? What are the core book(s) to play called?
Most of the versions in the OD&D category.3. which one of these two had non-human races as classes? both?
No. They're mostly compatible with few major rule changes. More akin to the difference between 3.0 and 3.5, although the nature of the changes was different.4. Are 1st and second edition very dramatically different? like 3rd and 4th were?
If someone can, more power to them. That seems like an incredibly difficult undertaking. I'd suggest that you take whatever anyone tells you in response with a grain of salt. Experiences differ greatly depending on when you played each version, who you played it with, what version you started with or played the most, and so on. I recall a startling conversation I had in the early 80s with some gamers from the next town over, and it quickly became clear that the AD&D they played was very different from the one we played. How much more varied are the experiences are the experiences of people picking up the old versions 20+ years later or gamers who've been playing older editions for just as long now looking at 3e and 4e?5. Can anyone give me a general feel or idea about these systems to help me decide if I'd like them?
In my experience, older versions tend to have simpler and quicker chargen, fewer choices in chargen, higher lethality, greater emphasis on player skill to defeat challenges, less concern with level-by-level balance between PCs, less reliance on magical items (though games vary widely between the Monty Haul and the mundane), and a general default position that your characters could try to do anything and the DM would adjudicate accordingly.
The oldest of OD&D is at once simple and arcane, probably because it's just so different from the modern game and because there are many wargamer assumptions built in. B/X and BECMI are much simpler and clearer. AD&D was intentionally more detailed and added many rules about many things. It's replete with tables and charts for all kinds of things. 2nd Edition AD&D streamlined that, but as time went on many optional rules built it back up.
One of the strengths of these older versions is the world building of the Gazeteers and the many, many modules. The other, for my money, is the settings: Planescape, Ravenloft, Spelljammer, etc.
This is a decent timeline