Android components
An android component is simply a piece of code that has a well defined life cycle e.g. Activity, Receiver, Service etc.
The core building blocks or fundamental components of android are activities, views, intents, services, content providers, fragments and AndroidManifest.xml.
Activity
An activity represents a single screen with a user interface. An application can have one or more activity. For example a contact app shows contact list might be an activity and where we create new contact it might be also other activity.
View
A view is the UI element such as button, label, text field etc. Anything that you see is a view.
Intent
Intent is used to invoke components. It is mainly used to:
Start the service
Launch an activity
Display a web page
Display a list of contacts
Broadcast a message
Dial a phone call etc.
Service
A service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different app, or it might fetch data over the network without blocking user interaction with an activity. Another component, such as an activity, can start the service and let it run or bind to it in order to interact with it.
Content Provider
Content Providers are used to share data between the applications.
Fragment
Fragments are like parts of activity. An activity can display one or more fragments on the screen at the same time.
AndroidManifest.xml
It contains informations about activities, content providers, permissions etc. It is like the web.xml file in Java EE.
Android Virtual Device (AVD)
It is used to test the android application without the need for mobile or tablet etc. It can be created in different configurations to emulate different types of real devices.