Flathub aka Flatpak aka xdg-app
Flathub is a central Flatpak repository.
Flatpak was formerly called xdg-app are the software available from Flathub as well as other sources.
Flatpak is a package format that works and is supported on most major linux distributions.
Snap aka Snapcraft is an alternative to Flatpak. They essentially are competing systems to provide the same thing. Both are part of a concept where version control of dependencies is handled by a great deal of redundancy. Rather than having binaries compiled for a specific linux kernel, distribution, and libraries, the pack contains redundant copies of all dependencies necessary for the specific compiled version of the binary itself.
Bloatware - an example where the efficiency of the linux architecture is bypassed for the purpose of simplifying the application developer's burden to create compatible binaries for various distributions -or- make it unnecessary for the end user to have to compile a software from source. Flatpak does allow developers to directly provide updates to users without going through distributions, and without having to package and test the application separately for each distribution.
Linux purists are opposed to bloatware packaged applications like Flatpak and Snapcraft. Also, the promise of compatibility across all distributions and versions has never been achieved, far from it. An example where the cure is possible worse than the disease itself.
On many distributions due to security restrictions Flatpaks run isolated from your system; they run with restrictions on what they can see and access and ship with their own libraries instead of using your system's libraries.
Flathub, a repository (or remote source in the Flatpak terminology) located at flathub.org, has become the central place for getting applications packaged with Flatpak.
To see if you currently have any flatpaks installed
flatpak list
Tired of seeing flatpak listings in the Mint Software Manager? Even when flatpak is not installed, the listings still show up in software manager. You can purge them with the following command:
sudo apt-get remove --purge libflatpak0
Only do this if you are not using flathub at all, and you no longer with to see flatpak software listed in the Mint Software Manager.