An Android Intent is an object that carries message from one component to another component with-in the application or outside the application.

There are three fundamental cases where one can use the Intents:

case 1:- To start an activity

The Intent describes the activity to start and carries any necessary data.

  Intent i = new Intent(this, TargetActivity.class);
   startActivity(i);

Data through intent can carried from one component to another in the following ways:

(1)- Sending primitive data through intent

 Intent i = new Intent(this, TargetActivity.class);
   i.putExtra("KeyValue", "ABC");
   startActivity(i);

   Intent intent = getIntent();     
    String myString = intent.getStringExtra("KeyValue");

(2)-Sending Bundle through intent

Intent mIntent = new Intent(this, Example.class);
Bundle mBundle = new Bundle();
mBundle.putString("Key1", value);
mIntent.putExtras(mBundle);
startActivity(mIntent);

Bundle extras = getIntent().getExtras();
    String value1 = extras.getString("Key1");

case 2:- To start a service

A Service is a component that performs the operations in the background without a user interface.

For this, we need to create the service by extending Service class as:-

package com.blogdemo;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;

public class DemoService extends Service {

  @Override
  public int onStart(Intent intent, int startId) {
  System.outprintln("This is my service")
    super.onStart(intent, startId);Y;
  }

  @Override
  public IBinder onBind(Intent intent) {
  //TODO for communication return IBinder implementation
    return null;
  }
  @Override
    public void onDestroy() {
        // TODO Auto-generated method stub
        super.onDestroy();
        Log.d(TAG, "FirstService destroyed");
    }
  } 

Now call this service from your main activity:-

package com.blogdemo;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;

public class AndroidServiceCallingActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        startService(new Intent(this, DemoService.class));
    }
}

It is also needed to declare your service in the manifest as:-

<service
  android:name="DemoService"
  android:icon="@drawable/ic_launcher"
  android:label="@string/service_name"
  >
</service> 

case 3:- To deliver a broadcast:

package com.blogdemo;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class SendBraodcast extends BroadcastReceiver{

    public static final String CUSTOM_INTENT = "TEST";

            @Override
            public void onReceive(Context context, Intent intent) {
                System.out.println("I am sending an intent");
                Intent i = new Intent();
                i.setAction(CUSTOM_INTENT);
                context.sendBroadcast(i);
            }
        }

Now create receiver class to receive the coming broadcasting intent

package com.blogdemo;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class ReceiveBroadcast extends BroadcastReceiver{

    @Override
    public void onReceive(Context context, Intent intent) {

        if (intent.getAction().equals(SendBraodcast.CUSTOM_INTENT)) {
                System.out.println("Here is my broadcasting Intent");

        }
    }
}

Types of Intents:-

Explicit Intents

These intents designate the target component by its name and they are typically used for application-internal messages - such as an activity starting an other activity. For example:

// Explicit Intent by specifying its class name
Intent i = new Intent(this, TargetActivity.class);
startActivity(i);

Implicit Intents

These intents do not name a target and the field for the component name is left blank. Implicit intents are often used to activate components in other applications. For example:

// Implicit Intent by specifying a URI
Intent i = new Intent(Intent.ACTION_VIEW, 
Uri.parse("http://www.finderd.com"));
startActivity(i);