A subset relation describes a relationship between two sets, where one set, called the subset, contains some or all elements of another set, known as the superset. If set A is a subset of set B, every element in A is also found in B. This is denoted as A ⊆ B. If a subset contains all elements of the superset, it is called an improper subset. Conversely, if it contains only some elements, it is a proper subset. For example, if 1, 2 is a subset of 1, 2, 3, then 1, 2 ⊂ 1, 2, 3.