Every 8 or so years they revamp the rules and release it as a new edition. Aside from few exceptions, they're mostly incompatible with each other and offer different rules for combat, skill usage and training, etc, and quality-of-life improvements.
3e (more specifically 3.5e) was extremely popular. There's now a ruleset called Pathfinder made by another company, Paizo, that continues the style of 3.5e and is mostly compatible with it (though it has its own core rulebooks and most people I know don't mix 3e and PF).
4e wasn't as well liked but it still had mostly good reception. It was very polarizing to many players. This obviously isn't the space for a debate about it but I think the biggest cause of complaints are the way the combat system was overhauled. Many people compare it more to a modern RPG video game rather than a pen and paper one.
5e has had a great reception from old and new players. It's rules are definitely closer to 3rd edition than it is to 4th. Though it brought in a lot of quality of life improvements from 4. For example while it makes sense that 'Hide' and 'Move Silently' are two separate skills, it just means a rogue has to split their skill points between two things. There's not many scenarios outside of comedy where you give a character 'Hide' but not 'Move Silently' or vice versa. 5e has merged both of these into a single Stealth skill.
All the editions are still considered Dungeons and Dragons though, and not separate games (of which there are many wonderful ones, some of which have their own 1e, 2e, 3e, an so on...)
Note that the main benefit of Pathfinder was breaking compatibility with the huuuuge number of splatbooks for 3.x, and in cleaning up the skills system.
Unfortunately, as Paizo seeks to grow its business and publish more content, they tend to keep adding special rules and things that kinda distort that original accomplishment.
Probably, but the skill system in 5e is totally different from 3.5e/Pathfinder. To me, 5e feels like it took the ideas behind the simplifications in Pathfinder and ran with them.
As an example, Pathfinder simplified lots of different combat rules under the Combat Maneuver system (CMB/CMD). 5e simplified them even more, simply as contested skill checks selected by GM fiat.
> as a DM/GM I generally threw out the rules as soon as they got in the way of a good narrative.
5e seems to be designed with that in mind. Bonuses are largely replaced with the "advantage/disadvantage" system which gives the DM much more leeway in determining bonuses (beside simplifying the mechanism overall). Skills are greatly simplified and reduced in number, and partially replaced with "Backgrounds" which are 100% DM fiat.
Example from my first 5e session. "Use Rope" is gone, but my character has the "Sailor" background. So by DM fiat I was able to identify the condition of a rope. No dice involved.
My gaming group stopped playing Car Wars after I min-maxed a trike vehicle to absorb damage with armored beer fridges. The beer fridges had more "hit points" per pound than regular armor. Every attempt to be clever after that felt like too much work.
Then there is Traveller, Runequest, Tunnels and Trolls, Car Wars (with Truck Stop etc) and rather a lot more.