What does the Cartesian product really mean. It is said that the Cartesian product of two sets $A$ and $B$ means the set of all ordered pairs $(a,b)$, where $a\in A$ and $b \in B$. But what is the use of defining it?
For what purpose it is defined in early days? What is the use of Cartesian product in defining a relation and a function. I understand clearly the meaning of function but feel difficulty in understanding the basis in which Cartesian product is defined.