Next.js is an open-source framework for building modern web applications with React, created by Zeit and now maintained by Vercel. It simplifies building server-rendered React applications, providing tools for server-side rendering and static site generation. It also includes features like automatic code splitting, dynamic imports, and hot module replacement to optimize performance and development experience. Additionally, Next.js has a plugin ecosystem for further enhancing development. Overall, it's a powerful and flexible framework focused on performance and ease of use.