I have just the book that will answer your questions!

There's MEL, and there are expressions. Expressions are nodes in your scene that get recalculated on the fly by Maya, and MEL scripts are programs that are usually triggered by the user interface and modify the contents of a scene.

You can use MEL scripts to modify your scene or set things up (like a chunk of a character rig, or a dynamics system) while expressions are part of the scene's evaluation and get executed during playback.

Back to the book, there are a ton of examples in here that illustrate how MEL and expressions can be useful. I recommend that you check it out:

https://www.melscripting.com/

-- Mark