Use the noun subset when you're talking about a group of things that fit in a larger category. For example, "new horror films" or "new comedies" are subsets of "new movies."
You can use the word subset any time you refer to small sets within larger ones, whether you're talking about Halloween candy, greeting cards, or classmates. However, the most common use of subset happens in mathematics, which is where the word comes from originally. Subset is short for "subordinate set," or a set that is entirely contained inside another set — it's subordinate, less important or smaller.