Counter is for frequency counting — missing keys return 0. defaultdict is for grouping or accumulating — missing keys auto-initialise from a factory.

Counter(words) builds a frequency map from an iterable and provides .most_common(n). defaultdict(list) lets you do d[key].append(val) without a KeyError on first access. Use defaultdict(int) for manual counting when you need more control than Counter provides. Both eliminate the dict.get(key, default) boilerplate throughout your code.