AppImage is a format to distribute an entire application as a single executable file. To run the app, an user simply needs to run this file — there’s no intermediate installation step. The application runs off this package, and it doesn’t place files on the base system. In addition, the AppImage file contains all libraries and files it needs to run, and this allows them to work on a large number of distributions. - What is an AppImage?

AppImageLauncher

Provide better desktop Integration (on the fly *.desktop).

You can integrate AppImages with a single mouse click, and manage them from your application launcher. Updating and removing AppImages becomes as easy as never before.

Now can be installed as an AppImage itself see AppImageLauncher-lite

$ appimagelauncher-lite-2.2.0-travis995-0f91801-x86_64.AppImage install # for first init

see also Install on Ubuntu or Debian

Tools

• appimage-builder - an AppImage to build other AppImage
• AppImageKit
• How can I examine the files inside an Appimage? - depends if you trust or not the Appimage, uncompress / or mount.
• How does an appimage persist settings between launches?

How to create an AppImage

Tutorial

Applied to Barrier, a simple Qt/Qml application.

• Creating Linux Apps That Run Anywhere with AppImage
• How to use AppImageTool to create package to run on older Linux

Appimage vs X / HN

Snap / HN - A very important aspect of snap that should have been noted in the article is the lack of user control over the a snap’s updating process; Users are not allowed to control when updates are applied, leading to a windows 10-like user experience - Snaps are universal Linux packages

Docker - Whereas Docker is focusing on containers for servers operated by “devops”, AppImage is focusing on desktop applications operated by end users.
Whereas Docker puts everything (besides the Kernel) into a container and shields it from the host, AppImages use the host OS and do not shield the application from it.
That being said, you could use AppImage for server applications, and you could bundle everything if you wanted. But then you could probably just as well use Docker.

FlatPack - Similar in many ways to Appimage except that it install via distribution app store. it uses Namespaces instead of AppArmour for sandboxing. The main difference is that Flatpaks can both use libraries included in the package and shared libraries from another Flatpak. Flatpak software is currently available in Arch Linux, Debian, Fedora, Mageia, Solus and Ubuntu. It is focused on desktops only. The developer of Flatpak is the Red Hat employee Alexander Larsson.

• Flatpak - an insecurity nightmare - Flatpak is a sandbox. And Flatpak is secure. The applications are not.