All 15 Iterators & the Iterator Protocol questions
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What exception signals the end of iteration, and how does a for-loop use it internally? What does the built-in next() function's optional default argument do? How do you manually exhaust and inspect an iterator without a for-loop? How do you implement a custom iterator by defining __iter__ and __next__? How does Python's iter() function work with a sentinel value as a second argument? How does the itertools module complement the iterator protocol? How does Python know whether to treat a for-in target as iterable, and what error occurs if it isn't? What is the relationship between generators and the iterator protocol? What is the difference between iter(obj) and using obj directly in a for-loop, in terms of what Python calls internally? What is the difference between __iter__ returning self versus returning a separate iterator object?
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What is the difference between an iterable and an iterator in Python?

Beginner

Answer

An iterable is any object implementing __iter__() that returns an iterator, such as lists, tuples, and dicts -- you can loop over it repeatedly. An iterator is an object implementing both __iter__() (returning itself) and __next__() (returning the next value or raising StopIteration) -- it's stateful and exhausted after one full pass.
nums = [1, 2, 3]         # iterable
it = iter(nums)          # iterator
print(next(it))          # 1
print(next(it))          # 2
Real-world example A file object is an iterator over its lines -- you can only read through it once before needing to reopen or seek back to the start.

Common follow-ups: Can an object be both an iterable and an iterator?;What happens if you call next() on an exhausted iterator?

Comprehensions & Generators;Magic Methods & Operator Overloading
All 15 Iterators & the Iterator Protocol questions