To begin with we have counting numbers. These numbers are also known as natural numbers and are denoted by a symbol 'N'. These numbers are obtained by adding one to the previous number. In other words once we know the first element we can obtain the elements following it by adding 1 to the successive elements. That is, we can have infinite (innumerable or a large number of them) number of them. Since natural numbers are infinite, we find it convenient to express them as a set. By set we mean a collection of any well-defined objects. Each individual object is also referred to as an element of that particular set. We should be clear that the concept of set can be used to represent infinite as well as finite number of elements. A simple example for finite number of elements would be that the vowels a, e, i, o and u can be expressed as a set. Generally the set of natural numbers is represented as:

                   N = {1, 2, 3,.............}.