Why You Shouldn’t Be Using a Task Killer with Android
I see this come up over and over again. People saying that a task is running in the background and they think it is killing their battery or hogging all of their memory. So their natural reaction is to download a program made to kill tasks. Here’s the thing… you are likely doing more harm than good by killing tasks that aren’t ready to end. I was the same way when I first got my CDMA Hero. There were tons of things running that I didn’t want so I just kept killing them. After a few weeks I realized that if I stopped using a task killer (and totally uninstalled it in fact) my phone actually began to run better! The applications would close themselves and things just seemed to be running better. I get that there may be short term benefits from clearing a task, but you should still take the time to read through this.
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. Here is the complete article at the Android Developer Site
Here is a more understandable article if you don't really understand this much