The document compares Flux and React.js, highlighting that Flux is an application architecture for building user interfaces developed by Facebook, while React.js is a JavaScript library for creating UI components. It discusses the advantages of using React.js with Flux, emphasizing improved UI/UX and unidirectional data flow, as well as detailing the core components of Flux architecture. Companies like Facebook and Instagram utilize this architecture to enhance their applications' performance and manage complex data flows.