React is a JavaScript library for building user interfaces. It allows developers to create reusable UI components and render them efficiently. React is typically used for building dynamic, interactive web applications. It provides a virtual DOM (Document Object Model), allowing developers to update the UI efficiently without manually manipulating the real DOM. On the other hand, Gatsby is a static site generator built on React. It uses React to build pre-rendered static websites and serves the user as HTML, CSS, and JavaScript files. Gatsby is designed to optimize performance and improve the user experience by reducing page load times and improving SEO. One of the key differences between Gatsby and React is that Gatsby provides additional features such as static site generation, server-side rendering, and support for GraphQL, which allows developers to query data from various sources and integrate it into their websites.