1. The Natural numbers are the numbers used for counting. Number theorists choose 1 as the first natural number, while others (i.e. set theorists) choose to start with 0.
2 & 3. A set is a collection of objects or numbers. The members of a set are called elements. Set A is a subset of set B if all the elements of set A are in B. In this case, N is a subset of W because all the elements in N are elements of W.