I've got a lot of experience with Java and C#, but I'm new to Android. I mainly use C# because I am enamored of the Control hierarchy. I love the plug-and-play of the ontology. I'm trying to understand the ontology in this new paradigm and I may have been given some false information.
With respect to Apps, that should be the largest component. Within the App, there may be several Activities. An activity can display a number of Fragments. AppWidgets appear to be a special case as they exist as a component of the App, but are shown on their own. And I was told that you can extend Buttons or ProgressBar to create your own components which again appear to be called Widgets.
As I said, I may have this completely wrong. Ideally I would like to create my own widgets which I can put on a Fragment, an AppWidget or an Activity; any of which I might compose into an App. All the online sources I've found only discuss Widget in the sense of an AppWidget? Was I given incorrect information? Can anyone clarify the ontology? Thanks
"Widget" is a bit of an overloaded term. You will probably have better luck if you search for tutorials on "custom Views" instead. I'll include a brief rundown of various terms and what they mean at the bottom.
A custom View is pretty much anything that extends the View class (or any of its subclasses) and isn't part of the framework. Custom views can be used wherever typical Views are expected, e.g. in layout files or directly constructed in Java. One thing to note: only certain Views can be used in an AppWidget because they are running in another process outside of your app. This means your custom Views cannot be used in AppWidgets. In my experience this tends not to matter too much.
App: An application. Contains components, which are defined in the manifest within the <application> tag.
Activity: One of the four application components. Nearly always has an associated UI, composed of a hierarchy of Views.
Fragment: A framework class that helps modularize your application's code and UI. Fragments can be attached to an Activity and can contribute some UI to the View hierarchy of the Activity. They are entirely optional; you don't have to use Fragments in your app, and you can attach a Fragment without it contributing any UI to the Activity.
View: A UI component, such as text (TextView) or images (ImageView). These are also referred to as "widgets", and you may notice the framework classes are found in the android.widget package. Some views contain other views, so that you can build a UI hierarchy; these will extend ViewGroup and are referred to as "view groups" or "layouts" more or less interchangeably.
AppWidget: Something the user can add to his or her homescreen. This is provided by the app, but is not one of the 4 application components mentioned previously (it is managed by an application component, namely a special subclass of BroadcastReceiver). Most people colloquially refer to these as "widgets" because it's shorter to say and launchers used that terminology as well, thus conditioning users to it.