There is a difference between them, actually. The one at the top is "up" and the one at the bottom is "back".

For example if you have a list of items and you click on one, then on that item you have related items and you click on one, the up button will take you to list where as the back button will take you to the previous item.

And you're right, you can not hide the taskbar.

You can find more information here: http://developer.android.com/design/patterns/navigation.html