There may be different server and desktop versions of the same distribution, and one may do
"rolling releases" while the other does "stable releases" ? True of Debian ?
32-bit and 64-bit versions, as on Windows.
Fully-encrypted system disk (using LUKS ?), or not.
Encrypted home directory, or not.
Various command-line shells: bash, zsh, fish, etc.
Are all versions/distributions of Linux free ?
Answer: most are free, but using a paid distribution may give you customer support, printed manuals, extra software.
Red Hat is not free ?
Do Windows and Linux use same filesystem structure ? So external disks and flash drives can be moved from one to the other easily.
Answer: Generally Windows uses NTFS, Linux uses EXT. But Linux handles many filesystem types.
You'll have to install the ntfs-3g package for NTFS. [But: Ubuntu 18 / Mint Tara automatically recognizes
Windows OS partition in a dual-boot system and mounts it; no package installation or other steps needed.]
Do Windows and Linux versions of major software (browsers, anti-virus, MS Office) have exactly the same feature-sets ? All add-ons that
work in a browser for Windows also work in same browser on Linux ?
Answer: usually yes, unless there is some funny situation such as an add-on that launches a companion application.
And for standalone applications, you could try running the Windows version in WINE.
If an application says it is available for the base I choose and the package manager I choose,
does that mean it will work with the end-distribution and desktop I choose ?
For example, a Debian application definitely will work on Linux Mint Cinnamon ?
Answer: no. Programs may not be available on both, or if they are available may be different versions.
Often you can force it to install, and it may work partly or fully.
If a distro/DE uses a particular windowing/application
framework/library (such as Kubuntu using KDE,
or Mint Cinnamon using Gnome), does that mean that applications based on some other
framework won't work, or will run badly, on that distro ?
Answer: no. All/most distros support the major windowing/application frameworks/libraries.
If you install a KDE-based application on Mint, the KDE framework/library will
get installed, and the application will run. Its UI might look slightly
different than those of apps based on a different framework.
One exception would be desktop widgets or applets. Those would be specific to a DE.
What happens to cloud services that you started through Windows and then access through Linux ?
Such as backups on IDrive, files on DropBox, etc.
Answer: usually they work, usually the service has an official Linux client.
But some, such as Google Drive, do not. There may be workarounds.
More-complicated things you can do:
Partition a disk into Windows and Linux partitions, so you can dual-boot (boot into either OS).
Boot Linux, and then sometimes run a Windows emulator (such as WINE)
to run some Windows applications.
Boot Windows, and then sometimes run a Linux command-line environment
(such as BASH) to do various operations.