先講結論：
Iterator 是透過一個class 內部宣告__iter__還有__next__(python2 是next)方法來達成可迭代(iterable)的物件(class)
Genetator 是透過yield簡單產生一個iterator的方法(method)

這是一般的iterator

class iterator(object):
    def __init__(self, start, stop):
        self.start = start
        self.stop = stop
    def __iter__(self):
        return self
    def __next__(self):
        if self.start >= self.stop:
            raise StopIteration
        current = self.start**2
        self.start += 1
        return current

這是用generator產生的方法

def generator(start,end):
    for i in range(start,end):
        yield i**2

官方document這樣形容generators
Generators are a simple and powerful tool for creating iterators
Anything that can be done with generators can also be done with class-based iterators as described in the previous section. What makes generators so compact is that the __iter__()and __next__() methods are created automatically.

之後用法大致上都相同，都具備有iterable的特性，雖然在用list()的時候感覺很像，是因為list有iterable的特性，但是iterator是用完就丟的概念，不能重複提取，好處是在處理時可以不用佔用到記憶體空間，像python2中xrange的用法