Python, what is the difference between indexing and slicing?

Question

Python

I want to update the list, but I don't understand what is the difference between indexing and slicing: why is 33 in the first version without square brackets, and in the second - with?

a = [1,2,3]
a[0:1] = [33]
a
Ответ: [33,2,3]

и 

a = [1,2,3]
a[0] = [33]
a
Ответ:  

a[0] returns this same element, that is, a[0]

  

Answer 1

index - one specific element
slice - a slice of elements in the specified range (may contain one element)