WebGL is a JavaScript API for rendering 3D graphics in web browsers without plugins. It is supported in modern desktop and mobile browsers depending on the graphics card. Three.js is a popular open-source WebGL framework that handles 3D scene graph management and rendering. It allows creating 3D scenes with meshes made from geometries and materials, adding cameras, and rendering with common patterns like box geometries, basic materials, and perspective cameras. Examples include simple cubes and panoramic views.