Task :

A task is a collection of activities that users interact with when performing a certain job. 

http://developer.android.com/guide/topics/fundamentals/tasks-and-back-stack.html 所以task 是一堆 activity 的集合。
Application 是 apk.
application 裡面是一堆 component (Activity, Service, BroadcastReceiver...)。
當這個 application 的 component 被叫起來時， android 就會啟動這個 application。