Editions, there have been a few.
1E is very much a different edition then OD&D, lots of changes, and OD&D is, well, quite an "odd" thing, actually.
2E is a different edition then 1E in part because they actually called it a second edition. Their term. And did totally rewrite and change all sorts of little and not so little thing. As noted, it led to quite viscous edition warring that could flare up years after the fact (as 1E rocks the flavor and avoids the bloat whereas 2E has some of the blandest core rules and led to a junkyard...).
Holmes basic is really its own, yes, odd, thing.
Moldvay cook is also its own, great clarifying edition of the rules. Mentzer, basically B.5, came too soon. Too soon. There is a lot of overlap, but people tended to move from B/X to BECMI/RC.
(Now, its true, you could use a B module with either B, or AD&D or OD&D. Of course, you could also use them with 3E with a little work, and (I certainly hope) with 5E with not too much work.)
2.5/Players options just about replaced the core rule books. I am pretty sure you could just about play without the PHB if you were doing a full 2.5 game. It was not marketed as such, and they still wanted you to buy the PHB and the other books. They did replace the monstrous compendium at this time, with the best MM ever. If you were a DM, you had to buy that book.
3E, obviously a new edition. Though with more 1e/2e detail then is sometimes appreciated.
3.5 was marketed as 3.5. With new core books that just did not tweak things but did more then that. And people thought it was not compatible. They where probably worse about this using B adventures with AD&D.
4E, yes, the biggest change of all. Or at least the hardest to mix and match with past stuff. It could be done, but with a fair amount of work.
4E essentials. Did some of us rant and rave on these boards about this really was a new edition, even as the WoTCs denied it. Yes, yes we did. You could buy and use 4EE products without any of the 4E "core books". And honestly, there would be no reason to get a DMG or MM after they came out. (or arguably before they came out). The PHB actually retained a lot of good options, as long as you had the dozens of pages of errata. Of course, we were all using DDI at that point so the books didn't matter. Which DDI you used could matter, on the other hand...
I like to thing that the 5E books that are coming out will be the ultimate edition. At least for me.