The Anatomy of a JavaScript Bookmarklet
Bookmarklets are a bit of JavaScript that you save as a bookmark and when visiting a page, if you click on that bookmark, the JavaScript will run on the context of the current page that is being viewed. This allows you to modify the current page in ways that the original author did not think off or allow. In essence, bookmarklets are a way to extend, modify, and enhance sites you are visiting.
Source: juixe.com