Android Fundamental Components

View
Views are user interface (UI) elements that form the basic building blocks of a user interface. A view could be a button, label, text field, or many other UI elements.  Views are also used as containers for views, which means that there's usually a hierarchy of views in the UI. In the end, everything you see is a view.

Activity
An activity is a user interface concept. An activity usually represents a single screen in your application. It generally contains one or more views, but it doesn’t have to. An activity is pretty much like it sounds—something that helps the user do one thing—and that one thing could be viewing data, creating data, or editing data. Most Android applications have several activities within them.

Intent
An intent generically defines an “intention” to do some work. Intents encapsulate several concepts, so the best approach to understanding them is to see examples of their use.

You can use intents to perform the following tasks:
􀀁 Broadcast a message.
􀀁 Start a service.
􀀁 Launch an activity.
􀀁 Display a web page or a list of contacts.
􀀁 Dial a phone number or answer a phone call.
Intents are not always initiated by your application—they’re also used by the system to notify your application of specific events (such as the arrival of a text message).

Intents can be explicit or implicit. If you simply say that you want to display a URL, the system will decide what component will fulfill the intention. You can also provide specific information about what should handle the intention. Intents loosely couple the action and action handler.

Content Provider
Data sharing among mobile applications on a device is common. Therefore, Android defines a standard mechanism for applications to share data (such as a list of contacts) without exposing the underlying storage, structure, and implementation. Through content providers, you can expose your data and have your applications use data from other applications.

Service
Services in Android resemble services you see in Windows or other platforms—they’re background processes that can potentially run for a long time. Android defines two types of services: local services and remote services. Local services are components that are only accessible by the application that is hosting the service. Conversely, remote services are services that are meant to be accessed remotely by other applications running on the device.  An example of a service is a component that is used by an e-mail application to poll for new messages. This kind of service might be a local service if the service is not used by other applications running on the device. If several applications use the service, then it would be implemented as a remote service. The difference is in startService() vs. bindService().  You can use existing services and also write your own services by extending the Service class.