Set

A (finite or infinite) collection of objects in which order plays no role and multiplicity is ignored. For example, the sets {1, 2, 3} and {3, 3, 3, 2, 2, 1} are equal, by definition.

Examples

 * My companies (a finite set) = {Google, Facebook, Microsoft, Twitter, Yahoo!}
 * My lucky numbers (an infinite set) = {7, 77, 777, 7777, 77777, 777777, 7777777, ...}
 * My wife (a singleton) = {Emma Watson}
 * My money (an empty set) = {}

Set Operations
I like the movies {Harry Potter, Star Wars, Star Trek}, whereas my wife, Emma, likes the movies {Harry Potter, The Bling Ring, Ballet Shoes}.


 * Intersection: {Harry Potter}
 * Union: {Harry Potter, Star Wars, Star Trek, The Bling Ring, Ballet Shoes}
 * Difference (Movies I like but my wife doesn't really like): {Star Wars, Star Trek}

Trivias



 * Venn diagrams are usually used to represent sets.
 * Multiset (or bag) is a set-like object in which order is still ignored, but multiplicity matters.
 * Georg Cantor is known to be the inventor of set theory.
 * Naive set theory contains a paradox known as Russell's Paradox. It can be resolved by means of ZFC (Zermelo–Fraenkel set theory with the axiom of choice).