Set theory is a branch of mathematical logic that studies sets, which informally are collections of objects. Although any type of object can be collected into a set, set theory is applied most often to objects that are relevant to mathematics. The language of set theory can be used to define nearly all mathematical objects. [wiki]
A set in Python is a collections of unique elements. Get your pen and paper and draw those venn diagrams to learn about set operations.
A set is a Many that allows itself to be thought of as a One.