Phone [Q] Greenify on Non rooted MotoG

I have a doubt from starting about what exactly does a task killer do and Greenify do.
I know that greenify force stops the applications which we list to be hibernated. Thought they are force stopped, the memory on the ram is not cleared if we notice, so the application opens quickly when reopened. But how is the ram not cleared even after force stopped?
Also what does a task killer do?? does it not force stop the application. Does it follow another procedure to kill the app which makes it more battery consuming?

One more thing which is more confusing is, what does clearing recent apps from the phone swiping does? From my observation it actually does the same as Greenify.
Can somebody please explain me all this.