def vs class

[pyzyx3qwerty]

In your second piece of code, there are three errors -

1. No : at the end of line 1
   
2. On line 2, you seem to have forgotten an 'l' - it should be
   

   if x=='furball':

3. On line 3, print shouldn't be having a capital p
   

However, anyways your code isn't expected to work as you just print Grumpy, not define it under a variable and call it, and so the expected error.
You can use this :

def Cat(x):
    if x=='furball':
      addition = 'Grumpy'
    return addition
 
print('Our latest addition:',Cat('furball'))