for them: Whatever you download is going to be "Debian Stable." Debian Stable is as fresh and up-to-date as it's ever going to be at this moment, but it will not change significantly in the future, because its goal is stability. You throw it on something you want to run for years and not crash.
If you don't mind your system crashing every once an a while because you like new stuff, you use "Debian Testing." There is no place to download this directly from Debian, although some third-parties (like Canonical with Ubuntu) distribute customized versions of it. The way you get a non-customized version is by installing Debian Stable, changing sources.list to point at Testing (which you can do as "testing" or by its nickname), then dist-upgrading. Debian Testing is being tested to be the next Stable.
"Debian Unstable" afaict is where individual pieces of software are being tested to go into Testing. Nobody should be running it unless they are contributing to Debian, although there are apt-get masters who know exactly what they're doing who will pull bleeding edge packages from unstable individually.
About the nicknames: Stable, Testing, and Unstable aren't releases, they're an indication of the current status of a release. Each release has its own goofy name. "Bookworm" has just moved from Testing to Stable. "Bullseye," which until just now was Stable, has now become "Oldstable."
Also important is the "[yourrelease]-backports" repo, which Stable users can add to take newer packages from Testing that are 99% certain not to mess with the stability of Stable. Stable + backports is a compromise between Stable and Testing for people who want new stuff that doesn't break things.
I'm sure most people know all of that, but 1) when it comes to things like this people are often afraid to ask because they're afraid they'll look stupid, and 2) the Debian website is very utilitarian and not marketing oriented, so there's no clear entry point for people who don't already know what they're looking for.
Trying to run a mix of stable and testing packages can be a pain as occasionally a package you want to bring in from testing will try to bring with it new system libraries, which in turn often conflict with the "stable" versions of packages (so you are forced to move a lot more of your system to "testing" packages than you originally wanted to fix this).
The key advantage (at least for me) of using the "backports" repositories is that it avoids this - packages are compiled against the "[yourrelease]" system libraries.
Ubuntu is based on Sid (Debian Unstable).
You can download Debian installers for Testing.
Usually nothing on Testing crashes, as such severe bugs are considered blockers to move a package form Unstable to Testing. (Of course once in a few years something slips through the testing period in Unstable. But it's than usually repaired within a few hours.)
People are running Sid. (Even I personally wouldn't recommend it.)
But one point of the parent I strongly support:
Debian's web page is a mess. I'm using this system for decades but still don't find anything on the Debian page without the help of some search engine. Also, when I need Linux related documentation I go to the Arch Wiki (and sometimes to the Gentoo docs), even as a Debian user. OTOH, you only seldom need docu, because Debian "just works" for the most part.
