I used to use greenify on my rooted Xperia U. The XDA page of Greenify states the difference between the task killer and greenify. Here is what the developer posted.

Unlike any "XXX Task Killer", your device never fall into the cat-mouse-game of stealthy-running and aggressive killing, which unnecessarily consumes much battery juice. Since all greenified apps will be put into hibernation until the next time you launch them, there is no need to "kill" them during the hibernation.

I know android dont need Task Killer App. I recently used Greenify on a non rooted phone in which I use hibernate shortcut to greenify app. Whenever I launch the shortcut. It will open the running apps App Info page and the Force Stop button will be automatically pressed (like an automated pearl script)

I didnt see this in my rooted phone. All I will see is Greenify had granted root permission and so and so apps are hibernated.

This confused me. Is there any difference between force stop and task kill. What is the differnce between Greenify and Task kill.

The thing about Task killer are, they kill the apps and if you are launching again the app has to start from the scratch and the task killer runs in background consuming battery. Is force stopping is similar to it? and yes Greenify do run in backfround.