QUOTE(gurpreetsingh @ Mar 14 2012, 05:00 PM)
that means to keep updates coming I need to check every apps?
I m not sure if I dont on my viber I can get call or not.
same goes with yahoo/gmail/talk.
I will test these individually and see how it goes.
There seems to be a lot of confusion regarding running apps, multitasking, background tasks and such.
When you launch an Android app, the task is
active/running. You use the app, play with the app, etc.
When you exit back to the home screen (or go to a different app), the app is
stopped. This means that the app is frozen and will not run/take up battery/etc. It also means the Android OS can freely kill that task if the Android OS needs more memory for other things (i.e. you opened up a really heavy game, you haven't gone back to that app in a while, etc).
So how does multitasking work? How does your app check for email in the background? This is done by something called
services. Services run in the background, you don't really see it anywhere on your screen. Even when you quit your app, it still has a service running in the background. It checks your mail even if you're not in the app, it checks for new messages when you're not in the app. If you're busy downloading something using your browser, and you go to a different app, your download still continues, because the browser is running it's own download service in the background.
An example to this would be WhatsApp. If you don't open the app, how does it check for new messages for you? It runs the WhatsApp background service to check for messages for you. You don't have to open the app to keep checking for new messages, it already runs by itself in the background. The Android OS takes care of what services to run, kill, etc automatically for you. That's why if you use a manual task killer (you shouldn't, btw), after a few minutes you'll notice the WhatsApp service running again.
This, of course, does not apply to ALL apps. It depends on whether the app developer has made such a feature available or not. Let's take Draw Something as another example. It doesn't have a background service, therefore it doesn't notify you if you have new drawings to guess.
tl;dr, to answer your question, you don't have to open viber/yahoo/gmail/gtalk to get notifications. The notification part of these apps are running in the background and are checking for new messages for you.
Also, regarding the ICS tasks button, all the tasks listed there are "frozen" apps... the apps themselves aren't running and aren't taking any CPU resources.
For better understanding, you can also read up on the Android Activity Lifecycle, but it's more technical and more for programmers:
http://developer.android.com/reference/and...tivityLifecycle