Difference Between List.append & List.extend

Theres is always confusion between list.append & list.extend.Lets see an example where a and b are two list as shown below

As shown above a has 3 element.when we extend a list element of b added at index 3,4,5.Now the length of the a a will be 6 or have 6 elements.But when we append a into b the b have 4 element as you can see full list b indexed at 3.