Flux is the application architecture that Facebook uses for building client-side web applications. It complements React's composable view components by utilizing a unidirectional data flow. It's more of a pattern rather than a formal framework, and you can start using Flux immediately without a lot of new code.
In this series we will explore the Flux Architecture for Facebook's React framework. From the basics through building a working example application, you'll learn the ins and outs of practical, real-world React development.
Be sure to check out the React Fundamentals series for an introduction to React.
We recommend watching the update ES6 version of this course.