Hammer.js is a JavaScript library for detecting touch gestures in web applications. It provides a modular architecture with a Manager class that coordinates input handling (touch, mouse, pointer events) and gesture recognition (tap, double tap, press, pan, swipe, pinch, rotate). The library normalizes different input types into a consistent gesture recognition system, manages gesture lifecycle states, and handles touch-action CSS property polyfilling. It features an event-driven architecture where recognizers emit events when gestures are detected, which are forwarded to registered listeners. The library has no runtime dependencies, relying solely on standard web APIs.