Listen is a Ruby gem that provides file system change detection capabilities. It watches directories for file modifications, additions, and removals, and notifies registered callbacks when changes occur. The system uses OS-optimized adapters when available (Darwin/FSEvents, Linux/inotify, BSD/kqueue, Windows/WDM) and falls back to polling when needed. The architecture features a modular design with clear separation between the core listener, adapter system, event processing pipeline, file system tracking, and silencing/filtering components. Key entry points include the Listen.to() method and the bin/listen executable.