Advanced Task Killer, i.e. ATK, is a very popular app, but widely misunderstood. Basically it allows users to kill running apps and background services. Whether this actually do anything is subject to debate. I have my own view on this and it's explained somewhere else. [ see link here ]
It works, though I only use it manually.
It is free, and there's a $4.99 donate version (ads-free). You should have it loaded, but do not use auto-kill.