The EventHandler plugin is a type of web-service that sends data asynchronously from a client to the server. Unlike traditional web-services, the EventHandler can send asynchronous events to the client from the server. Traditional web-services are constrained by the limitations of the HTTP protocol. The EventHandler may be considered more useful than traditional web-services for firmware developers since many embedded devices require real time update of the user interface.
See the getting started guide for an introduction to the EventHandler.