Terminologies 

Here, we will first learn some terminologies for creating android APP.

.apk file - Application package file - used to install application
.dex file - Compiled code file
Activity - you can place your UI with setContentView(View).
Intent - its an operation to be performed like start new activity to launch activity.

to create an android app we need to know the activity life cycle and activity stack.

For more information on this you can visit below links.

Activity lifecycle - http://developer.android.com/training/basics/activity-lifecycle/index.html

Activity stack - http://developer.android.com/guide/components/tasks-and-back-stack.html

you can always refer developr.android.com