说明：此笔记为“Android开发”学习视频的笔记，链接如下：http://open.163.com/movie/2010/1/8/D/M79HE97C3_M79HEQJ8D.html
一, Activity: A represnetation of a user doing something,
例子：显示联系人列表并且选中一个联系人
Activity的三种状态：Stopped， Paused，Active
二, Service: an application or an applicaton component that doesn‘t have a user interface
例子：媒体播放器
三, Broadcast Receiver:
1,Receive and react to "broadcasts"
2,Don‘t have a user interface, but can invoke Activities that do
3,Create by extending the Android BroadcastReceiver class
4,Example: An application wants to be notified when a photo is taken on the device
四，Content Provider
1，Make‘s an application‘s data avaliable to other apps, and can also be used by the defining app itself
2, Example: Make contacts avaliable to other apps
五，Intents： An “intent” is an asynchrounous message sent from one Activity to another