В моем понимании, которое, как я признаю, может быть ограничено в этом отношении, и поэтому я спрашиваю здесь, хотя не существует строгого или стандартизированного набора определений или различий, общепринятая точка зрения имеет, как это часто бывает, случай, постепенно принявший форму, поскольку компьютеры прогрессировали за эти годы.
Если я правильно понимаю предмет, основная идея - в настоящее время состоит в том, что расширения, приложения, сервисы, темы и плагины, которые не установлены по умолчанию, являются различными типами дополнений. Ниже приведены некоторые примеры, а также быстрая диаграмма, которую я построил для иллюстрации своего описания:
¹ Ожидается, что есть некоторые смутные моменты, когда общие разграничения не так ясно известны: например, будет ли шрифт считаться расширением системы или, поскольку он будет добавлен в набор способных функций браузера, будет ли он рассматриваться возможно тип плагина? Лично, поскольку использование указанного шрифта (ов) обычно не относится только к браузеру, я не думаю, что это будет. Хотя точки зрения других могут отличаться. Несомненно, есть и другие серые зоны ...
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.
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:
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?