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 See more basics on Android 

All the components are activated by a message called Intents. Intents bind individual components to each other at runtime, whether the component belongs to your application or another. An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component—an intent can be either explicit or implicit, respectively.
An intent is an abstract description of an operation to be performed. Intents are send to the Android system via a method call, e.g. via the startActivity() method. They can be said as a message which allow android components to request functionality from other components.

Intent Filters:Activities and intent receivers include one or more filters in their manifest to describe what kinds of intents or message they can handle or want to receive.An intent filters lists a set of requirements, such as datatypes,action requested and URI format that the intent or message must fulfil.

Types Of Intents:
Implicit Intents :
Implicit Intents have not specified a component; instead, they must include enough information for the system to determine which of the available components is best to run for that intent.

Button browserview= (Button)findViewById(R.id.bview);
browserview.setOnClickListener(new OnClickListener() {
 public void onClick(View v) {
   Intent browserintent = new Intent(Intent.ACTION_VIEW, Uri.pars("http://www.http://eazyprogramming.blogspot.in/"));
   startActivity(browserintent);
}
});

Explicit Intent:
Explicit Intents have specified a component, which provides the exact class to be run. Often these will not include any other information, simply being a way for an application to launch various internal activities it has as the user interacts with the application.

Button openclass= (Button)findViewById(R.id.bopenclass);
openclass.setOnClickListener(new OnClickListener() {
 public void onClick(View v) {
   Intent openintent = new Intent(FirstActivity.this,SecondActivity.class));
   startActivity(openintent);
}
});

public class MainActivity extends Activity {
}

Services :
          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 : Running an alarm in background
A service is implemented as a subclass of Service.

Content providers :
          They manages a shared set of application data.Data can be stored in SQLite Database,File systems,web or any other medium where application can access.
Content Providers allows to modify and use these stored data.For example : storing users contacts details,where proper permission one can get the contacts.
A content provider is implemented as a subclass of ContentProvider.

Broadcast receivers :
          They responds to systems  broadcast announcements.Some of the native Broadcast are volume control, light, switch on and off, screen on and off,bluetooth etc.A broadcast receiver is just a "gateway" to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.
A broadcast receiver is implemented as a subclass of BroadcastReceiver.

Who Activates A component ?
         The above components are activated by a message called Intents.
Intents bind individual components to each other at runtime, whether the component belongs to your application or another.
An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component—an intent can be either explicit or implicit, respectively.

         The Content Provider, is not activated by intents. It is activated when there is request from ContentResolver. These Resolvers handles direct transaction with the providers so they donot need further any method calls. This leaves a layer of abstraction between the content provider and the component requesting information (for security).

What is an AndroidManifest file ?
          This is a root directory of Android application where all the components are declared.Other than this they also have

• user permissions the application requires, such as read and write external storage
• Declare the minimum API level
• Declare hardware and software features such as bluetooth or a printer
• API libraries etc...