Daniel Dilger, writing for Apple Insider:

In 2014, Apple introduced App Extensions, a new architecture for building app components that could be distributed with an app, and designed specifically to add insert new functionality into core OS features on both iOS and macOS.

The following year, Apple introduced Content Blockers as a new class of App Extensions specially targeting Safari, supporting the ability to prevent the downloading of any defined content, including display ads, images, navigation elements, popups, scripts, fonts, style sheets, media files, cookies, or essentially anything on a web page.

On both macOS Sierra 10.12 and today’s El Capitan 10.11.5 (when Safari 10 is installed), Safari will support App Extensions built from a combination of JavaScript, CSS and native code written in Objective-C or Swift.

And:

More importantly, the new App Extensions architecture enables developers to distribute Safari Extensions as part of their app through the App Store.

This opens a brave new world for developers, who can now build Safari extensions, custom designed to extend their app’s reach through the browser, distributing app and extension side-by-side in the Mac App Store.