Hash Set: stores unique elements. Use when you only care about presence. - "Have I seen this number?" - "Is this string in the dictionary?" Hash Map: stores key-value pairs.
Use when you need to associate data with keys. - "How many times does each number appear?" - "What index did I first see this number at?" In Python, set is a hash set, dict is a hash map. In C++, unordered_set and unordered_map.