To activate an Activity an intent object with information of action being requested and URI of the data to be acted upon is passed to Context.startActivity() or Activity.startActivityForResult().

A service can be passed by passing an intent object to Context.startService(). To establish an ongoing connection between the calling component and a target service Context.bindService() is called by passing an intent object.

Similarly and intent object is passed to Context.sendBroadcast(), Context.sendOrderedBroadcast(), or Context.sendStickyBroadcast() to activate a service.

Activities

You can say Activity is a set of UI components available for users to be used , at some specific instant of time. For example; an Activity in image viewer application can be image and its description and navigation available. Moving from one activity to another is accomplished by having the current activity start the next one. Each visual component in an activity is called view and is derived from base View class.

Service

Service runs in background for an indefinite period of time and doesn't have an interface. Each service extends Service Base class.

Service usually is involved in background work, while the user is busy with Activity. let us take an example of a car racing game.It may consists of several Activities like one displaying game status and controls to move the car. And there can be a service playing the background music.

Broadcast receivers

A broadcast receiver is a component which receives a broadcast message and takes some action based on the message. Broadcast receiver can start an Activity in response to Broadcast message of battery low, SMS received, or alarm clock time out. Each broadcast receiver extends BroadcastReceiver class.

Content providers

A content provider makes a specific set of the application's data available to other applications. The data can be stored in the file system, in an SQLite database. Each content provider extends ContentProvider base class.

These days the BIG thing  in the smart phone's race  is the introduction of Andriod Operating System. Behold II will launch with Andriod 1.5 and eventually update to Andriod 2.0, sometime in 2010.

It is yet to see that how Samsung creates some space for Behold II when  HTC and Motorolla, just a couple of days back introduced two most talked about smart phones HTC Driod Eris and Motorolla Driod. HTC runs of Windows Mobiles and Motorolla Driod is the first phone that runs on Andriod 2.0. So for Andriod enthusiasts, Motorolla has introduced just the thing they want. Some critics are of the view that it makes no sense that consumers will be keen in a set which is already three updates behind.

It looks like the Behold II won't update to 2.0 until 2010 which begs the conspiracy theory that Google and Verizon cooked up a deal to leave the Motorola DROID as the only Android 2.0 device for 2009. What do you guys think? [via tmonews]

The other big feature in the set is Samsung's own Touchwiz UI. To get an idea what the UI is all about, look at the video at the end. To make navigation simple, Samsung's UI consists of a cube with applications on all sides of the cube that eventually makes it easy to jump from one application to the other. But is it really required? That is the big questions.

So all eyes are on Samsung that how, in the midst of a storm of Andriod high end phones, Behold II makes its place.

Huawei's phone would be targeted for low-end customers. Further details aren't made public.