In my understanding—which I admit may be limited in this regard, hence why I'm asking here—although there is no strict or standardised set of definition(s) or distinction(s), a generally accepted perspective has, as is often the case, gradually taken form as computers have progressed over the years.

If I comprehend the subject correctly, the basic idea¹ nowadays is that extensions, apps, services, themes, and plugins that are not installed by default are distinct types of addons. Some examples, as well as a quick diagramme I constructed to illustrate my description, are below:

enter image description here

  • Extensions
    • Firefox Extensions
    • Chrome Extensions
    • Opera Extensions
    • Etc...
  • Apps
  • Services
  • Themes
    • Chrome Themes
    • Firefox Themes
      • Personas (first changed to "Background Themes", now just "themes")
      • Complete Themes
  • Non-Default Plugins
    • Certain programmes give the option to install plugins into one's browser to extend said programme'(s') functionality into said browser.

¹ Expectedly, there are some vague points where common demarcations are not quite so clearly known: for example, would a font be considered a system extension or, because it is something that is added on to the browser's set of capable functions would it be considered perhaps a type of plugin? Personally, because usage of said font(s) is typically not specific to just the browser, I don't think it would be. Although others' viewpoints may differ. There are undoubtedly other grey areas as well...

  • Themes are look and feel changes only. That much is certain.
    – cybernard
    Aug 13 '15 at 23:54
  • FireFox apps are only for the FireFox operating system, and yes there is such a thing.
    – cybernard
    Aug 13 '15 at 23:58
  • Extensions are dependant on the host application. IE without firefox a firefox extensions does nothing.
    – cybernard
    Aug 14 '15 at 0:02
  • What a wonderfully formatted post. @cybernard So many comments you might as well post an answer :)
    – MC10
    Aug 14 '15 at 0:02
  • @cybernard: Yes, I know that themes are look and feel only. I never denied as much. Also, yes, Firefox apps are most certainly possible for non-Firefox-OS platforms. You can use many of them in Firefox for Windows, for instance. Aug 17 '15 at 19:05

Each term has its own meaning which is not limited for world of browsers. This is my attempt do show merits of these terms:

  • App(lication) - stand-alone task-focused program. It runs in its environment, for example in operating system or browser.

  • Extension - program which is not standalone, but adds new capabilities to existing application

  • Plugin - like extension, but usually with clearly visible boundaries. While extension can modify many underlying functions of the system, plugin is understood as extension for single well-defined task. But it depends, see below the list.

  • Service - technically, program which runs on the background and serves requests. Socially, a platform serving content and/or building community.

  • Theme - visual modification of existing user interface.

Important: these terms have their meanings indicated by their names. But they are vague and used based on customs/feelings in particular platforms/communities. For example, addon, add-in, plugin and extension can pretty much mean the same thing. And this list is not limited, for example, a marketing department of some painting program can come with term "splash-in" or "PowerHelper". So even marketing can have its role here, too. Term service is used in two different contexts as I indicate. Only apps and themes have somewhat fixed meaning. Themes were sometimes called skins. Maybe apps can be distinguished as small and handy programs when compared to program suites. Calling Adobe Illustrator or Microsoft Excel an "app" looks odd (at least to me), it is still more an application than app. But do not try to generalize differences between addon, add-in, plugin and extension, but instead, learn which community/platform/application uses which term, form example:

  • Outlook has add-ins
  • GIMP, IrfanView of Photoshop have plugins
  • Firefox has addons
  • Visual Studio has extensions
  • Microsoft Management Console has snap-ins
  • etc.

If some platforms use multiple types at once, the main thing is to learn difference between them, but again, it is only in context of that platform. For example, Joomla! CMS has components, modules and plugins, all covered by term "extensions". But it has also "languages", what is just another type of extension. You see, everybody uses what fits their world, the same term often has different meanings on different platforms.

On the other hand, I do not hesitate to use any common term when communicating informally: Is there a good cloud-based to do list plugin for Outlook, Chrome or Firefox? Did you try to start it with all plugins disabled? But that functionality is available only through extensions, right?

  • GIMP has plug-ins, extensions and themes, just to make this a bit more challenging. Aug 14 '15 at 16:32
  • @MichaelSchumacher – Firefox has multiple types of stuff, too – it was just a simplification from my side.
    – miroxlav
    Aug 14 '15 at 20:07
  • @miroxlav: I'm aware that it gets a lot more complicated when you start talking about any specific software. Hence my emphasis on a general set of distinctions. Aug 17 '15 at 19:09
  • Also, as you mentioned clearly in your 2nd full paragraph, an "app" is typically not the same as an "application", though they can be in certain contexts. In terms of addons, though, it is rarely, if ever, the same thing. Typically, an application is standalone whilst an app is supplementary & not standalone. Aug 17 '15 at 19:11
  • And yes there are synonyms produced either by differences between subcultures, companies, or professions, but most of this would merely be different manifestations of the same category, all essentially just the same category in multiple states. Like so. I guess you could call it a quantum category. ;) xD Aug 17 '15 at 20:22

Themes are look and feel-changes only. That much is certain.

Firefox apps are only for the Firefox operating system, and yes there is such a thing.

Extensions are dependent on the host application. A Firefox extension does nothing without Firefox, for example.

Chrome Apps are only for the Chromebooks which run Google's own operating system ChromeOS.

  • (1) I know; I never said themes were anything other than look-and-feel changes. (2) Many Firefox apps are compatible with Firefox for Windows. (3) I also never said extensions are independent. (4) Chrome apps can run on Windows just fine. I do it all the time. You just need the Chrome App Launcher. Aug 17 '15 at 20:25
  • And Chrome. Aug 17 '15 at 20:25

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