【Activity 与 Task】
A task is a collection of activities that users interact with when performing a certain job. The activities are arranged in a stack (the back stack), in the order in which each activity is opened.
A new activity is, by default, launched into the task of the activity that called startActivity(). It's pushed onto the same back stack as the caller. However, if the intent passed to startActivity() contains theFLAG_ACTIVITY_NEW_TASK flag, the system looks for a different task to house the new activity. Often, it's a new task. However, it doesn't have to be. If there's already an existing task with the same affinity as the new activity, the activity is launched into that task. If not, it begins a new task.
参考：http://android.xsoftlab.net/guide/components/tasks-and-back-stack.html