What is set theory?

Set theory is a mathematical problem that studies sets, which are collections of objects

The branch of mathematics which deals with the formal properties of sets as units (without regard to the nature of their individual constituents) and the expression of other branches of mathematics in terms of sets. A Set is an unordered collection of objects, known as elements or members of the set. An element 'a' belong to a set A can be written as 'a ∈ A', 'a ∉ A' denotes that a is not an element of the set A.