Android doesn't like to waste ram. So Android will store applications frequently used in free ram to speed up the phone. Once ram is needed, the prestored apps are cleared out of ram so quickly that the user doesn't even realize they were there.
Ram does not use battery, processor use does. The apps in ram shouldn't be using the processor but it does happen occasionally if the app is poorly coded. Reading reviews and looking for apps showing up in the battery usage when it has barely been used is a way to identify these poorly coded apps.
That said, some manufactures are intentionally coding apps to stay in ram regardless of how often they are used. The phrase is "lock in memory." Only essential apps (launcher, alarms, mms, etc) should be locked in ram. However some manufactures are locking nonessential apps in memory (slacker radio, bing, city id, etc). They do it for money from the company that makes the app. It is called bloatware. Moto has a bad rep for doing this. Say the phone has 512mb of ram, well Moto will lock slacker radio in memory no matter what, even if the ram is needed. Slacker radio performs better at the cost of everything else on the phone. It is as if that ram didn't exist to any app except slacker radio. The solution to this is rooting and installing a custom rom.
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