Having trouble understanding ContextActionService and UserInputService, can anyone help?

Hello.

I've been looking into ContextActionService and the UserInputService for a few days now. It's hard to wrap my head around even one of the two. I always see people say they prefer one or the other but they don't explain the differences, and I'm having trouble grasping on how to use them.

And yes, I have checked on the official ROBLOX wiki as well as the few updated tutorials of the two services on YouTube.

So, my main questions are:

1. What are the differences between the ContextActionService and UserInputService?
2. Which of the two would be better, or "preferred"?
3. How do I learn them how to use them? I'm having a lot of trouble with it.

Any help will be greatly appreciated.

Thank you! :D