The Web Push API lets users of your Web app receive push notifications at any time—even in the background; that is, even when your Web app is not currently running in the foreground on the user’s device (or even when the browser on the user’s device is not currently running). It use Service Workers to handle messages sent using common push services, and to allow your Web app to react to the push notifications it receives.
For more information about the Web Push API, see the actual Push API spec and the articles Using the Push API and Push Notifications on the Open Web. The MDN article Using Service Workers is also relevant.
The IETF standards that describe the Web Push protocol and its message encryption scheme are really useful to understand how to build an application server that sends push notifications to clients and how to encrypt push message payloads.
