ArsTechnica published an article on Safari 5's Extensions

Safari 5 includes a new secure browser extension system that is similar in many respects to Chrome's extensions and Firefox's Jetpack plugin. Extensions are created completely in JavaScript, HTML, and CSS, and Apple takes great pains to differentiate extensions from plug-ins. Safari plug-ins written in native code are solely for supporting new media types within the browser (Flash, Quicktime, et cetera) whereas extensions are for adding new functionality to the browser itself.

The first iteration of the extension system allows developers to add features to a number of carefully controlled places within the application. Developers can create toolbars, add items to contextual menus, manipulate windows and tabs, add toolbar buttons, and manipulate and extract content within webpages themselves. Extensions can also selectively inspect and filter resources and deny them from loading.

  Feature: Getting to know Safari 5's Extensions