Roughly speaking, a set is a collection of objects. The objects are called the members or the elements
of the set.
Set theory is the basis for mathematics, and there are a number of axiom systems for set theory; von
Neumann-G¨odel-Bernays (NBG) and Zermelo-Fraenkel-Choice (ZFC) are the most well-known. I’m going
to take a somewhat informal approach to avoid unnecessary complexity.