[Galaxy S III Q&A, Help & Troubleshooting] krabman:
I posted this before and its a cut and paste but I don't recall where I got it anymore. I was trying to help explain how android uses memory and why task killers can often hurt more than they help unless the person using them understands android memory management and doesn't work against it. I have deleted my own conclusions on task killers and leave here just the basics on how android uses memory. The main thing you should come away with is that if you run out of memory you probably have a problem, most likely a rogue app. At any rate here is the cut and paste.....

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.

An activity has essentially three states:

It is active or running when it is in the foreground of the screen (at the top of the activity stack for the current task). This is the activity that is the focus for the user's actions.

It is paused if it has lost focus but is still visible to the user. That is, another activity lies on top of it and that activity either is transparent or doesn't cover the full screen, so some of the paused activity can show through. A paused activity is completely alive (it maintains all state and member information and remains attached to the window manager), but can be killed by the system in extreme low memory situations.

It is stopped if it is completely obscured by another activity. It still retains all state and member information. However, it is no longer visible to the user so its window is hidden and it will often be killed by the system when memory is needed elsewhere.

If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling its finish() method), or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

The foreground lifetime of an activity happens between a call to onResume() until a corresponding call to onPause(). During this time, the activity is in front of all other activities on screen and is interacting with the user. An activity can frequently transition between the resumed and paused states - for example, onPause() is called when the device goes to sleep or when a new activity is started, onResume() is called when an activity result or a new intent is delivered. Therefore, the code in these two methods should be fairly lightweight.

A few conclusions.....

Most services (while possibly running in the background) use very little memory when not actively doing something.

A content provider is only doing something when there is a notification for it to give. Otherwise it uses very little memory.

End cut and paste

The thing is android is designed to use as much memory as possible. This allows it to start paused activities using less resources overall which saves both time and cpu cycles and also saves battery. Even when you reboot your phone and look at the memory usage android does not need all the memory it has already used. It has called some processes in anticipation of their use but these are inactive and it will shut them down if you need those resources which happens in time measured by cpu cycles and this is very very fast. If you cannot run an application without running out of memory can pretty much figure you have something which isn't playing well with others.