I'm new to android development and I'm trying to learn the proper terminology.
Are activities, services, broadcast receivers and content providers classes or classes instances (objects)?
(I've seen activities referred as classes or objects depending on the book or tutorial).
Android is based on Java , and Activity, Services,Broadcast Receivers are all classes which extends a Common Root class Object.
So you can implement all the Object Oriented Programming Concepts with these classes.
Below is the Hierarchy of an Activity in Android system, similarly it is applicable for others too.
public class Activity extends ContextThemeWrapper implements LayoutInflater.Factory2, Window.Callback, KeyEvent.Callback, View.OnCreateContextMenuListener, ComponentCallbacks2 java.lang.Object ↳ android.content.Context ↳ android.content.ContextWrapper ↳ android.view.ContextThemeWrapper ↳ android.app.Activity
For more info you can check developers.android.com