>>> append_list = []
>>> append_list.append(1)
>>> append_list.append('a')
>>> append_list.append(['ssss'])
>>> append_list
[1, 'a', ['ssss']]

>>> extend_list = []
>>> extend_list.extend(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
>>> extend_list.extend([1])
>>> extend_list.extend(["a", [3, 4] ])
>>> extend_list
[1, 'a', [3, 4]]