Phaser is an open-source, lightweight JavaScript framework for building 2D games for web and mobile. You define game objects (sprites, groups, physics bodies), handle input (keyboard, mouse, touch), manage state, and animate. Phaser handles rendering, physics, collision detection, and audio. Phaser is perfect for 2D games: platformers, top-down shooters, puzzle games, and interactive fiction. It's not for 3D games (use Babylon.js or Unreal for that). It's excellent for web-first game development.