WebSocket
WebSocket is a protocol providing full-duplex communication channels over a single long lived TCP connection.
Comparing to HTTP, WebSocket is more suitable for real-time communication, such as online games, trading systems, and chat applications.
Socket.IO
Socket.IO is an event-driven library for real-time web applications. It enables real-time, bi-directional communication between web clients and servers. It consists of two components: a client, and a server. Both components have a nearly identical API.
It is built on top of the WebSocket protocol and can fallback to HTTP long polling in case the WebSocket connection cannot be established.