From Google Devs:
Android was designed from the ground up as an operating system (OS) for mobile devices. Its built-in application and memory-management systems were engineered with battery life as one of the most critical concerns.
The Android OS does not work like a desktop operating system. On a desktop OS, like Windows, Mac OS X, or Ubuntu Linux, the user is responsible for closing programs in order to keep a reasonable amount of memory available. On Android, this is not the case. The OS itself automatically removes programs from memory as memory is needed. The OS may also preload applications into memory which it thinks might soon be needed.
Having lots of available empty memory is not a good thing. It takes the same amount of power to hold "nothing" in memory as it does to hold actual data. So, like every other operating system in use today, Android does its best to keep as much important/likely-to-be-used information in memory as possible.
As such, using a task manager/killer to constantly clear memory by killing apps is strongly NOT RECOMMENDED. Generally speaking, you should only "End" applications if you see one which is not working correctly.
