Three.js is the JavaScript library that wraps WebGL in a friendly API for 3D rendering in the browser. Ricardo Cabello started it in 2010; the project now powers 3D visualizations on news sites, product configurators (Apple's pioneering work, Audi configurator), portfolio websites with eye-catching effects, and the rising "spatial web" use cases.
The React Three Fiber library has become the default way to use Three.js in React applications — it expresses Three.js scenes as React components with the same declarative model. Most modern Three.js courses cover both the imperative API and the React Three Fiber wrapper, plus drei (a community-maintained collection of helpers).