Mainly android consist of following building blocks....
Activities
The building block of the user interface is the activity. You can think of an activity as being the Android analogue for the window or dialog in a desktop application. While it is possible for activities to not have a user interface, most likely your “headless” code will be packaged in the form of content providers or services, like the following described.
Content Providers
Content providers provide a level of abstraction for any data stored on the device that is accessible by multiple applications. The Android development model encourages you to make your own data available to other applications, as well as your own—building a content provider lets you do that, while maintaining complete control over how your data gets accessed.
Intents
Intents are system messages, running around the inside of the device, notifying applications of various events, from hardware state changes (e.g., an SD card was inserted), to incoming data (e.g., an SMS message arrived), to application events (e.g., your activity was launched from the device’s main menu). Not only can you respond to intents, but you can create your own, to launch other activities, or to let you know when specific situations arise (e.g., raise such-and so intent when the user gets within 100 meters of this-and-such location).
Services
Activities, content providers, and intent receivers are all short-lived and can be shut down at any time. Services, on the other hand, are designed to keep running, if needed, independent of any activity. You might use a service for checking for updates to an RSS feed, or to play back music even if the controlling activity is no longer operating.
