Tasks、 activity 及 activity stack
http://www.cnblogs.com/yaozhongxiao/archive/2013/10/12/3365345.html
Android中Task、Activity、BackStack(栈)的关系
http://blog.csdn.net/benweizhu/article/details/7614189
Android Fragment---与Activity生命周期的协调
http://blog.csdn.net/think_soft/article/details/7272869
Android Activity切换和过渡动画(解决back键后退失效问题)