Socket.io is a JavaScript library that enables real-time bidirectional event-based communication between web clients and servers. It works across all modern browsers and mobile devices, simplifying development of realtime applications involving messaging, notifications, and collaboration. Key features include supporting multiple transport mechanisms like websockets, flash sockets, and AJAX long-polling just fallback, providing a simple API, allowing multiplexing with namespaces, and integrating session support via handshakes.