A set is a collection of objects, which are called elements. Generally there is a rule that is assigned, which can be used to determine whether a particular object belongs to a set or not. For example, we could speak of the set of even single digit numbers, which is a collection of numbers 2, 4, 6, 8.
A set is denoted by a symbol like F, or A and so on. The set A is a sub set of F if all elements of A are also elements of F. The elements of a set can also be identified by symbols, with or without subscripts or superscripts.