Socket.IO is an event-driven module or library, and, as you probably guessed, is based on EventEmitter. Everything in Socket.IO works with events. An event is triggered when a new connection is made to the Socket.IO server and an event can be emitted to send data to the client.
The Socket.IO server API differs from the Socket.IO client API. However, both work with events to send data from client to server and vice versa.