ReactJS is a popular JavaScript library for building user interfaces that allows developers to create reusable UI components. It has gained widespread popularity due to its simplicity, consistency and ability to quickly render complex UIs. Some of the key benefits of using React include its ease of use, support for custom components, fast rendering and SEO-friendly design. It also has a large developer community contributing tools and support. Overall, ReactJS is well-suited for developing large-scale web applications with timely data updates and rich interactive user interfaces.