A subset is a set in which every element is also a member of another set. If all elements of set B are in set A, then B is a subset of A. The subset concept is key in comparing and organizing data.
\[ B = \{3, 8, 9\} \Rightarrow B \subseteq A \quad \text{(B is a subset of A)} \\ C = \{1, 5\} \Rightarrow C \nsubseteq A \quad \text{(C is not a subset of A)} \]
Set B contains elements that all exist in set A, so it is a subset. But set C contains elements not in A, so it is not a subset.