GatsbyJS is a site generator that allows you to build modern, fast and secure apps and websites using React, GraphQL, and other tools. It focuses on developer experience with batteries included and features like hot reloading. Popular sites using Gatsby include reactjs.org, airbnb.io, and figma.com. Gatsby gets data from various sources and delivers sites via services like S3, Netlify, and GitHub Pages. Developers can install Gatsby globally, generate a new Gatsby site, and develop locally while previewing changes in real time.