The ServiceWorker specification allows scripts to cache resources and handle all resource requests for an application even when a network isn't available. To put it in a different way, the ServiceWorker browser feature enables developers to build applications that work offline, overcoming the limits of HTML5 Application Cache (AppCache) by giving developers a set of primitives to handle every HTTP call on a Website, as well as controlling installation and updates for the cache rules.
Sounds cool, right? Well, it is. Interested in going in-depth? A few resources:
Be sure to check this post I have pubblished on ProgrammableWeb
The first draft of ServiceWorker spec
A detailed explaination by Alex Russell
Another great post by Jake Archibald