Learn in the textbookSets and Logical Operations
DefinitionA set (集合) is a collection of things for which it is clearly decided whether each one meets a given condition.
ExplanationEach individual member of a set is an element, and when every element of set X is also in set Y, X is a subset of Y. From two sets you can form the union, which gathers the elements in either one, and the intersection, which gathers only the elements in both. The elements of the whole that are not in a given set form its complement.
AnalogyLike a club roster: whether your name is on it decides whether you are in. A group such as "tall people," with a vague standard, is not a set.
Memory hookRemember ∪ as the shape of U, the first letter of Union, and ∩ as that shape turned upside down.