What is the difference between class and instance variables?

Question

Python

I want to catch the essence, the meaning in class instance variables?
I understand that a class variable is something like a global variable and all instances can use it, while a class variable is a local variable and only an instance of the class can use it? In what cases, then it is worth setting a class variable?

class Mag:
    lvl = 100

    def __init__(self, name, rank):
        self.name = name
        self.rank = rank

    def spell(self, enemy) -> None:
        if enemy == 'human':
            print('You can`t kill a human')
        else:
            print(f"{self.name} make a spell on {enemy}!")

Thanks in advance for the clarification!

Answer 1

I advise you to read the book by M. Lutz, it seems, the second volume.
There, just "by atoms", the author analyzes the principles of work of classes, instances and OOP as a whole.
If the pens got to OOP, then you need to know in sufficient detail how this paradigm works, everything is explained in the book.