1. 每个app 都有一个自己的 linux 进程；
2. 每个进程都在自己的虚拟机里执行
3. 两个app 可以跑在一个进程，一个vm里
4. android app 四大组件：activity，content provider,      services, broardcast receivers
5. Content Resolver 激活 Content Provider

• You can start an      activity (or give it something new to do) by passing an Intent to startActivity() or startActivityForResult() (when you want      the activity to return a result).
• You can start a      service (or give new instructions to an ongoing service) by passing an Intent to startService(). Or you can      bind to the service by passing an Intent to bindService().
• You can initiate      a broadcast by passing an Intent to methods like      sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().
• You can perform      a query to a content provider by calling query() on a ContentResolver.

From <http://developer.android.com/guide/components/fundamentals.html>