Offical description of various catalog file types /names (Windows)?

I’m not sure if there’s a reference by Adobe online, but for some of them…

CATALOGNAME.lrcat

This is the catalog itself. So never delete this unless you actually do want to delete a catalog forever, but in that case you don’t just want to delete this file, but the entire containing folder, because if you delete the catalog you’ll also have no use for the other files in the same folder.

CATALOGNAME Previews.lrdata\ [hidden system folder]

This is the cache of already built previews. It starts out small and grows as you view more images, because every time you bring a new photo into view in any module other than Develop, a preview is built. If you have a large number of images (tens of thousands or more), the Previews cache can become many GB in size, which can be a problem on small volumes. But it’s OK to throw it out because Lightroom Classic will simply start over and generate new previews for any images you view.

CATALOGNAME Smart Previews.lrdata\ [hidden system folder]

This is also a previews cache, but of Smart Previews only. It is also OK to throw this out at any time, but you should first stop and think about why they exist, because Smart Previews are typically created on demand…in other words, at some point you decided you needed them. If you still need them for some photos but not others, you can delete them from selected photos by choosing Library > Previews > Discard Smart Previews.

After writing all that, I realized that there actually is a listing of each file type you asked about, and additional types that appear only when Lightroom Classic is open, starting on page 460 in the book Adobe Lightroom Classic: The Missing FAQ by Victoria Bampton.

I don't know what those files/folders are, but I can tell you how to relate to them: ignore them 😉

These are all "helper" files that will speed up working, but they are not essential to operation. If you delete them, they get rebuilt as needed.

The .lock file is somewhat special. That prevents multiple instances of the catalog to be open at the same time. If you see that with Lightroom closed, delete it. It should only be there when the application is open - but it may be left behind if the application crashes. If you can't open the catalog, this is often the reason.

I have a backup regime to synchronize the catalog to another machine independently from the built-in backup. This is set up to exclude all these extra folders (they take a long time to copy). Only the catalog itself is synchronized.