I don't know the difference between class variables and instance variables. Can anybody give me an example?

Thank you so much:)

Here you have an example of a class name Animal and some instance names (dog, cat, cow) ...

class Animal:
    class names by convention are capitalized
    # category is global to the class and is available to all 
    # instances of class Animal
    category = "animal"

    def __init__(self, animal, sound):
        __init__() is the 'constructor' of the class instance
        when you create an instance of Animal you have to supply
        it with the name and the sound of the animal
        'self' refers to the instance
        if the instance is dog, then
        name and sound will be that of the dog
        'self' also makes name and sound available
        to all the methods within the class
        self.name = animal
        self.sound = sound
    def speak(self):
        the first argument of a class method is always self
        this method needs to be called with self.speak() within
        the class, or the instance_name.speak() outside the class
        print( "The %s goes %s" % (self.name, self.sound) )

# create a few class instances
# remember to supply the name and the sound for each animal
dog = Animal("dog", "woof")
cat = Animal("cat", "meeouw")
cow = Animal("cow", "mooh")

# testing ...
# now you can call each animals function/method speak()
# by simply connecting instance_name and speak() with a '.'

# you can also access variables associated with the instance
# since cow is the instance
# self.sound becomes cow.sound

# category is a class variable common to all istances

"""my result -->
The cow goes mooh
The cat goes meeouw
The dog goes woof

According to advice from The_Kernel (thanks)
category" is considered a class variable,
self.name and self.sound are instance variables.

A class variable is shared across instances of the class, while an instance variable is unique to that instance of the class. Consider the following example:

class Foo:
    name = "FOO"
    def __init__(self, value):
        self.value = value
mine = Foo("bar")
mine_2 = Foo("taz")

print mine.value, mine.name
print mine_2.value, mine.name

Foo.name = "BAR"

print mine.value, mine.name
print mine_2.value, mine.name

#output is:
# bar FOO
# taz FOO
# bar BAR
# taz BAR