A set is a well-defined collection of distinct objects.

The objects that make up a set (also known as the elements or members of a set) can be anything:

numbers, people, letters of the alphabet, other sets, and so on.

There are two ways of describing, or specifying the members of, a set.

One way is by intensional definition, using a rule or semantic description:

EX:  A is the set whose members are the first four positive integers.

       B is the set of colors of the French flag.

The second way is by extension – that is, listing each member of the set.

An extensional definition is denoted by enclosing the list of members in curly brackets:

C = {4, 2, 1, 3}

D = {blue, white, red}