WebSockets provide a new way to communicate with the server. Traditional communications by a webpage is request-response: the browser sends a request for information to the server, which then sends back a response. Each request and response uses a new connection, and that connection is closed after the response is returned to the client. As you can imagine, this is a poorly performing method because of the time spent creating and closing each connection. Also, such communication cannot be two way because both client and server cannot talk simultaneously, and the server does not easily maintain a connection to the client.
WebSockets uses a different approach in that it provides duplex, or two-way, communication between the server and client. Both parties can communicate at the same time, as in chatting or instant messaging clients. It also limits connection creation and disposal so that it occurs only once rather than with every message. It is essentially a TCP-based protocol that enables two-way communication to occur over a single connection.
Source: Exam Ref 70-486- Developing ASP.NET MVC 4 Web Applications