ReactJS is a JavaScript library for building user interfaces using reusable and interactive UI components. It uses a virtual DOM for rendering UI components and follows a unidirectional data flow architecture. Major companies using ReactJS include Facebook, Instagram, AirBnB, and Khan Academy. ReactJS advantages include its use of components, JSX syntax, and debugging tools. Components are the basic building blocks and use props, state, and lifecycles. Flux is a common architecture used with ReactJS that involves a central store for managing data in a unidirectional flow.