<human>:  What is set theory?
<bot>: Set theory is a branch of mathematics that studies sets, which are collections of objects. It is used to study the properties of sets, as well as relationships between sets, such as unions, intersections, and differences. Set theory is also used to define functions and sequences, and to analyze the structure of space and time. Set theory is an important part of mathematical logic, which is a formal language used to reason about and solve problems.