Oh God........... Not another one of these threads.
Unfortunately, as long as there are new users there will be these threads.
A couple of pieces of advice for the OP. First, of course, learn to use the "search" button. But don't feel too bad, you're not alone.
Second, if you're determined to get a task killer, take a look at "System Panel." It's a task killer for people who hate task killers. The UI is very informative (in real time) about the apps that are actually "running" versus those that are "
inactive!" and simply sitting in memory because the o/s has had no reason to push them out.
You'll also find that most "inactive" apps started when you booted your phone, used a few cpu cycles for a few seconds, and then quietly went "inactive."
Plus, it provides useful information about such things as current clock speed and historical info about use of your memory.
If, in fact, you find that you have an active application that is serving no purpose or doesn't go inactive on its own, System Panel will allow you to kill it. Otherwise, as long as you have at least 30 mg of free memory, killing applications that are doing nothing but sitting in memory does nothing for you except require that the o/s goes to additional work to activate them when it needs to do so.
P.S. Of course if you're obsessive about it and just hate having Google force an Amazon MP3 app on you, you can always get a little thrill by killing it even though it's doing no harm.