An application usually contains multiple activities.
Each activity should be designed around a specific kind of action the user can perform and can start other activities.
For example, an email application might have one activity to show a list of new email.
When the user selects an email, a new activity opens to view that email.
An activity can even start activities that exist in other applications on the device.
For example, if your application wants to send an email, you can define an intent to perform a "send" action and include some data, such as an email address and a message.
An activity from another application that declares itself to handle this kind of intent then opens.
In this case, the intent is to send an email, so an email application's "compose" activity starts (if multiple activities support the same intent, then the system lets the user select which one to use).
When the email is sent, your activity resumes and it seems as if the email activity was part of your application.
Even though the activities may be from different applications, Android maintains this seamless user experience by keeping both activities in the same 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.
The device Home screen is the starting place for most tasks.
When the user touches an icon in the application launcher (or a shortcut on the Home screen), that application's task comes to the foreground.
If no task exists for the application (the application has not been used recently), then a new task is created and the "main" activity for that application opens as the root activity in the stack.
When the current activity starts another, the new activity is pushed on the top of the stack and takes focus.
The previous activity remains in the stack, but is stopped.