The multitasking bar, double tap the home button, shows three classes of apps:
1. Recents, those that were used in order of use, that are not running.
2. Those that are suspended or sleeping.
3. Those that are running in the background.
The difficult part is knowing which apps fall into which category. The only ones that one really needs to terminate are the apps running in the background as they use cpu and memory resources. The cpu management system is very good at prioritizing cpu usage, a part of the Unix operating system from back in the 80's. With the iOS form of the system the user cannot access control over the prioritization routine. So you are stuck with what has been programmed into the iOS.
Tasks running in the background use resources and battery charge. Unless you really need them, that is, they are doing something for you that you want done, go to the multitasking bar and terminate them. I do that daily with all apps just to make sure I have full resources and to maximize battery life.