A Set theory is the branch of mathematics that studies sets, which are essentially a collections of objects. These objects are called the elements or members of the set. Objects can be anything: numbers, people, other sets, etc. For instance, 7 is a member of the set of all odd numbers. Clearly, the set of odd numbers is infinitely large; there is no requirement that a set be finite.

Although any type of objects can be collected into a set, set theory is applied most often to objects that are relevant to mathematics. Because of its generality, set theory forms the foundation of every other part of mathematics. Sets are of great importance in mathematics; in fact, most mathematical objects (numbers, relations, functions, etc.) are defined in terms of sets. Nowadays it is known to be possible, logically speaking, to derive practically the whole of known mathematics from a single source – "The Theory of Sets".

Set theory was introduced by George Cantor in 18th century, later it was developed by John Venn and De Morgan. Cantor has given the basic idea about set theory, Venn has given the pictorial form of sets and De Morgan proposed laws on the sets.