A unique aspect of the Android system design is that any application can start another
application’s component. For example, if you want the user to capture a photo with the device camera, there's probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You don't need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera application that captures a photo. When complete, the photo is even returned to your application so you can use it. To the user, it seems as if the camera is actually a part of your application.
When the system starts a component, it starts the process for that application (if it's not
already running) and instantiates the classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your application's process. Therefore, unlike applications on most other systems, Android applications don't have a single entry point (there's no main() function, for example).  
Because the system runs each application in a separate process with file permissions that
restrict access to other applications, your application cannot directly activate a component from another application. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the component for you.
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应用程序组件(Application Components)
Android的一个重要特征是，一个应用程序可以使用的其他应用程序的元素（如果应用程序允许）。例如，如果你的应用需要滚动图像列表中，而另一个应用程序已制定了切实可行，并允许他人使用，则可以滚动列表直接调用来完成作业，而不是开发你的应用不吸收或其他应用程序的链接，它只是当需求开始的其他应用这部分。部件有应用四种不同的类型，每个类型服务于不同用途，生命周期不同，如何定义创建和销毁。有四种组件： 应用程序组件英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_40616.html