There is a fundamental difference between add-ons and apps.

Apps are individual applications that extend Hubzilla’s functionality for the user (channel). Many of them also come with their own interface (though not all… AP, for example, does not) for the user to use them or, in some cases, to configure them. Some apps are part of Hubzilla’s core and are always available for the user to activate (install), whilst many are not part of the core and are made available via add-ons.

Add-ons are installable extensions to Hubzilla’s core functionality. They can make their functionality available to users via apps, but can also extend functionality without any action on the user’s part (in which case they do not come with an app… they take effect for everyone simply by being activated by the admin).

In short: add-ons extend Hubzilla’s core functionality. Apps allow users to activate and/or access core functions or functions provided by add-ons.

Ultimately, only the admin needs to know the difference, as ordinary users do not come into contact with add-ons at all.