ReactJS is a JavaScript library for building user interfaces. It uses a component-based approach to build modular and reusable UI components that make code more maintainable and boost productivity. Some key advantages of React include its simplicity, ability to easily create reusable UI components, support for one-way data binding that makes the data flow clear, ability to render on both client and server sides, and strong performance due to its use of a virtual DOM. It is used by many large companies including Facebook, Instagram, Netflix, and Airbnb.