Linux

They take Ubuntu LTS and add their software on top of it. Ubuntu is the base. It's stable because it's unchanging, you only get security and bug fixes, no new versions except the Neon additions. It's implemented like it is because starting with a complete and freely available distro like Ubuntu is a lot less work than building from scratch.

I think it qualifies as a distro by any current definition, but maybe not one they expect to be in general use. It seems to be quite popular despite that. I've never used it though so I can't comment on how it is.

KDE Neon is new KDE with old Ubuntu. Kubuntu is old KDE with new Ubuntu

Everyone else has answered your other questions, but let me just say that I've used KDE Neon (while distro hopping) it's both beautiful and functional. The latest KDE developments are really solid and intuitive, and based on their development updates I've seen on Mastodon and elsewhere, it seems like they're really starting to understand their userbase.

I.E. they understand that most people want to double click icons to launch them even though almost the entire development team prefers single click. They're not doing the tunnel-vision dev thing where they force what they love on users even though the vast majority of the userbase prefers the opposite.

Edit: It looks and feels gorgeous, and even similar alternatives like Kubuntu kind of feel a little worse to me. Then again, I'm a serious Cinnamon / KDE fan, so other peoples' mileage may vary.

KDE is a Desktop Environment.

"KDE Neon" is the latest Ubuntu LTS, but with fresher KDE packages and support straight from the KDE team.

Usually the KDE version would be a few releases older than what the KDE Neon distro offers.

KDE Neon was designed for someone who wants Kubuntu but with the latest KDE features. Just stick with Kubuntu if you’re a noob, or better yet, go with Debian and choose the KDE version. Your experience will be better and it will be more stable.

If you want to get the latest and greatest, go with Arch, but that requires (or at least should require coughManjaro*cough) the ability to read technical documents to fix random issues that will occasionally pop up when you are using the freshest and newest software.

It was good. Never had an issue with it. I'll generally stick with Plasma desktop centric distros because of the features that they offer that Gnome desktops don't. I switched to Manjaro KDE not because I had an issue with Neon, I just simply was bored and wanted to learn new console commands for the basics after years and years of apt.

1. Ubuntu with newest version of KDE stuffed into it. In short it's more for developers to test latest KDE versions in a relatively stable and easily maintained environment but gained popularity since it's newer than Kubuntu and doesn't enforce snaps.

2. By "stable base" they mean Ubuntu. The package systems (apt and snap), the kernel choice and configuration, the background services are what make the "back end" of a distro and Ubuntu is an example where this backend is given LTS (long term support) maintenance and thorough testing, even if that also means using older versions of the backend software in question. KDE Neon uses this because if something goes wrong with the distro it's likely their fault and not some backend stuff unrelated to KDE. Ubuntu is also good for building spinoff distros as most of the work is done for you - You just make your tweaks to a "minimal" ubuntu ISO and share it!

3. GnomeOS is probably the main example of a distro designed for testing rather than daily use. It's maintained by GNOME for testing their changes and can also be used for daily stuff if you want. Not sure what else "quasi distro" could mean, but there are a bunch of Ubuntu-based distros (And spinoffs to other big distros) that exist on a spectrum of "almost identical" to "completely redesigned".

4. I used it for a few months. It's pretty good! You get brand new KDE features on top of a familiar ubuntu system. It gives you the feeling of a bleeding edge UI experience but you don't risk the software driving that new UI being super buggy or breaking things. That being said, some users (like myself) prefer different software managers or don't like Ubuntu for other reasons. KDE-Neon as a distro in itself is a pretty solid choice, though.

I've used kubuntu and neon in the past. The issue I ran into was kubuntu not having the latest KDE software, and it wasn't available in back ports. I tried switching to neon but it's based on the LTS version of Ubuntu so the kernel was pretty old, it didn't have great support for my hardware.

I switched to tumbleweed and have been loving it since.

You can use distros like Arch or Debian as a base and install KDE to it as a desktop. Many distros based on Arch or Debian come with a desktop pre-installed. Like KDE. My favorite distro with KDE is Garuda KDE lite (it removes the visuals from regular Garuda). It uses Arch as a base, just like Steam OS and works well for me to game on. Steam OS for desktops is something I think will become popular once the official release from Valve comes.

Imo Kde neon serves as a test bench for whoever want to try the latest KDE DE directly from the kde team, it is not maint for daily driver use though I tried it for a while and it was pretty reliable.

Kubuntu is an ubuntu with kde that is maint to be used for daily driver usage.

KDE neon is a distro. It's the long-term support version of Ubuntu as the base with the latest stable KDE software on top.

I've personally used it for years because I love the KDE software suite and that is pretty much the sole focus here.