We've already seen Ubuntu running on tablets and smartphones, but not in any official capacity. Rumors had it that Canonical would be making a serious push into the tablet space in early 2011, but that effort never materialized, or at least was never acknowledged. Still, Unity has some finger-friendly streaks and Oneiric added ARM support -- so it's not much of a stretch to see the popular Linux distro on your mobile devices. Well, at the Ubuntu Developer Summit, Canonical founder Mark Shuttleworth made that move official by issuing a challenge to the Ubuntu community to start pushing beyond the traditional PC form factor. Few details were given, but Shuttleworth did say that he believes the time is right for the OS to start making moves into the tablet, phone, in-dash infotainment and smart TV spaces. There were no products to announce, but Shuttleworth was confident the OS would be ready and in shipping consumer electronics by the time version 14.04 arrived in April of 2014. Though, we're sure some prototypes will start showing up sooner rather than later.

By default, every application runs in its own Linux process. Android starts the process when any of the application’s code needs to be executed, and shuts down the process when it’s no longer needed and system resources are required by other applications.

A content provider is active only while it’s responding to a request from a ContentResolver. And a broadcast receiver is active only while it’s responding to a broadcast message. So there’s no need to explicitly shut down these components.

Activities, on the other hand, provide the user interface. They’re in a long-running conversation with the user and may remain active, even when idle, as long as the conversation continues. Similarly, services may also remain running for a long time. So Android has methods to shut down activities and services in an orderly way:

An activity can be shut down by calling its finish() method. One activity can shut down another activity (one it started with startActivityForResult()) by calling finishActivity().
A service can be stopped by calling its stopSelf() method, or by calling Context.stopService().

Components might also be shut down by the system when they are no longer being used or when Android must reclaim memory for more active components.

If the user leaves a task for a long time, the system clears the task of all activities except the root activity. When the user returns to the task again, it’s as the user left it, except that only the initial activity is present. The idea is that, after a time, users will likely have abandoned what they were doing before and are returning to the task to begin something new.

There is no need for an app to do the task of killing processes in the memory since, the processes staying in memory doesn't use battery, but killing the apps takes up CPU and hence battery.