Concept of Activity
一个app从不同地方打开,可能在不同位置,比如Email app,从social media app打开是writing email page. Activity 就是用来解决这样不同entry points的问题。
Activities are normally loosely bound with each other.
Activities are registered in AndroidManifest.xml files.
Intent用于Activity之间的通讯。Details about Activity and Intent: http://www.cnblogs.com/feisky/archive/2010/01/16/1649081.html