If you double tap the home button it brings up the multitasking bar. You can then tap and hold your finger on any app until they all begin to wiggle, then tap the X to close one of them.
Take a look at this Apple doc for more info -> iOS: Understanding multitasking
That won't help improve battery life as much as the ideas on this page -> Apple - Batteries - iPhone
The list is not open apps but recently used apps. More on multitasking...