It is a generic entry point for event-driven background processing in the Web Platform that is extensible by other specifications. It works through the provision of a browser-based offline processing capacity, allowing a website to store documents, and other resources.
The draft specifying how Service Workers should be implemented in browsers, co-edited by Google Engineer Alex Russell and Jungkee Song of Samsung Electronics, describes a method that enables applications to take advantage of persistent background processing, including hooks to enable bootstrapping of web applications while offline.
A Web page is treated like a desktop application, with a shell stored locally to serve as the initial user interface if a site is offline, then a Service Worker provides the user with cached pages, or other information and functionality to peruse until access is restored.
The service will be coming to Google Chrome later in the year, while Mozilla is also planning to implement Service Workers for Firefox.